The Story of Germ Life Part 4

There are still other methods by which nitrogen is being lost from the food cycle. First, we may notice that the ordinary processes of vegetation result in a gradual draining of the soil and a throwing of its nitrogen into the ocean. The body of any animal or any plant that chances to fall into a brook or river is eventually carried to the sea, and the products of its decomposition pa.s.s into the ocean and are, of course, lost to the soil. Now, while this gradual extraction of nitrogen from the soil by drainage is a slow one, it is nevertheless a sure one. It is far more rapid in these years of civilized life than in former times, since the products of the soil are given to the city, and then are thrown into its sewage Our cities, then, with our present system of disposing of sewage, are draining from the soil the nitrogen compounds and throwing them away.

In yet another direction must it be noticed that our nitrogen compounds are being lost to plant life--viz., by the use of various nitrogen compounds to form explosives. Gunpowder, nitro-glycerine, dynamite, in fact, nearly all the explosives that are used the world over for all sorts of purposes, are nitrogen compounds. When they are exploded the nitrogen of the compound is dissipated into the air in the form of gas, much of it in the form of free nitrogen. The basis from which explosive compounds are made contains nitrogen in the form in which it can be used by plants.

Saltpetre, for example, is equally good as a fertilizer and as a basis for gunpowder. The products of the explosion are gases no longer capable of use by plants, and thus every explosion of nitrogen compounds aids in this gradual dissipation of nitrogen products, taking them from the store of plant foods and throwing them away.

All of these agencies contribute to reduce the amount of material circulating in the food cycle of Nature, and thus seem to tend inevitably in the end toward a termination of the processes of life; for as soon as the soil becomes exhausted of its nitrogen compounds, so soon will plant life cease from lack of nutrition, and the disappearance of animal life will follow rapidly. It is this loss of nitrogen in large measure that is forcing our agriculturists to purchase fertilizers. The last fifteen years have shown us, however, that here again we may look upon our friends, the bacteria, as agents for counteracting this dissipating tendency in the general processes of Nature. Bacterial life in at least two different ways appears to have the function of reclaiming from the atmosphere more or less of this dissipated free nitrogen.

In the first place, it has been found in the last few years that soil entirely free from all common plants, but containing certain kinds of bacteria, if allowed to stand in contact with the air, will slowly but surely gain in the amount of nitrogen compounds that it contains. These nitrogen compounds are plainly manufactured by the bacteria in the soil; for unless the bacteria are present they do not acc.u.mulate, and they do acc.u.mulate inevitably if the bacteria are present in the proper quant.i.ty and the proper species. It appears that, as a rule, this fixation of nitrogen is not performed by any one species of microorganisms, but by two or three of them acting together. Certain combinations of bacteria have been found which, when inoculated in the soil, will bring about this fixation of nitrogen, but no one of the species is capable of producing this result alone. We do not know to what extent these organisms are distributed in the soil, nor how widely this nitrogen fixation through bacterial life is going on. It is only within a short time that it has been demonstrated to exist, but we must look upon bacteria in the soil as one of the factors in reclaiming from the atmosphere the dissipated free nitrogen.

The second method by which bacteria aid in the reclaiming of this lost nitrogen is by a combined action of certain species of bacteria and some of the higher plants. Ordinary green plants, as already noted, are unable to make use of the free nitrogen of the atmosphere It was found, however, some fifteen years ago that some species of plants, chiefly the great family of legumes, which contains the pea plant, the bean, the clover, etc, are able, when growing in soil that is poor in nitrogen, to obtain nitrogen from some source other than the soil in which they grow. A pea plant in soil that contains no nitrogen products and watered with water that contains no nitrogen, will, after sprouting and growing for a length of time, be found to have acc.u.mulated a considerable quant.i.ty of fixed nitrogen in its tissues The only source of this nitrogen has been evidently from the air which bathes the leaves of the plant or permeates the soil and bathes its roots This fact was at first disputed, but subsequently demonstrated to be true, and was found later to be a.s.sociated with the combined action of these legumes and certain soil bacteria. When a legume thus gains nitrogen from the air, it develops upon its roots little bunches known as root nodules or root tubercles. The nodules are sometimes the size of the head of a pm, and sometimes much larger than this, occasionally reaching the size of a large pea, or even larger.

Upon microscopic examination they are found to be little nests of bacteria In some way the soil organisms (Fig 27) make their way into the roots of the sprouting plant, and finding there congenial environment, develop in considerable quant.i.ties and produce root tubercles in the root. Now, by some entirely unknown process, the legume and the bacteria growing together succeed in extracting the nitrogen from the atmosphere which permeates the soil, and fixing this nitrogen in the tubercles and the roots in the form of nitrogen compounds. The result is that, after a proper period of growth, the amount of fixed nitrogen in the plant is found to have very decidedly increased (Fig 25 E).

This, of course, furnishes a starting point for the reclaiming of the lost atmospheric nitrogen. The legume continues to live its usual life, perhaps increasing the store of nitrogen in its roots and stems and leaves during the whole of its normal growth.

Subsequently, after having finished its ordinary life, the plant will die, and then the roots and stems and leaves, falling upon the ground and becoming buried, will be seized upon by the decomposition bacteria already mentioned. The nitrogen which has thus become fixed in their tissues will undergo the destructive changes already described. This will result eventually in the production of nitrates. Thus some of the lost nitrogen is restored again to the soil in the form of nitrates, and may now start on its route once more around the cycle of food.

It will be seen, then, that the food cycle is a complete one.

Beginning with the mineral ingredients in the soil, the food matter may start on its circulation from the soil to the plant, from the plant to the animal, from the animal to the bacterium and from the bacterium through a series of other bacteria back again to the soil in the condition in which it started. If, perchance, in this progress around the circle some of the nitrogen is thrown off at a tangent, this, too, is brought back again to the circle through the agency of bacterial life. And so the food material of animals and plants continues in this never-ceasing circulation. It is the sunlight that furnishes the energy for the motion. It is the sunlight that forces the food around the circle and keeps up the endless change; and so long as, the sun continues to s.h.i.+ne upon the earth there seems to be no reason why the process should ever cease. It is this repeated circulation that has made the continuation of life possible for the millions and millions of years of the earth's history. It is this continued circulation that makes life possible still, and it is only this fact that the food is thus capable of ever circulating from animal to plant and from plant to animal that makes it possible for the living world to continue its existence. But, ah we have seen, one half of this great circle of food change is dependent upon bacterial life.

Without the bacterial life the animal body and the animal excretion could never be brought back again within the reach of the plant; and thus, were it not for the action of these micro- organisms the food cycle would be incomplete and life could not continue indefinitely upon the surface of the earth. At the very foundation, the continuation of the present condition of Nature and the existence of life during the past history of the world has been fundamentally based upon the ubiquitous presence of bacteria and upon their continual action in connection with both destructive and constructive processes.

RELATION OF BACTERIA TO AGRICULTURE.

We have already noticed that bacteria play an important part in some of the agricultural industries, particularly in the dairy.

From the consideration of the matters just discussed, it is manifest that these organisms must have an even more intimate relation to the farmer's occupation. At the foundation, farming consists in the cultivation of plants and animals, and we have already seen how essential are the bacteria in the continuance of animal and plant life. But aside from these theoretical considerations, a little study shows that in a very practical manner the farmer is ever making use of bacteria, as a rule, quite unconsciously, but none the less positively.

SPROUTING OF SEEDS.

Even in the sprouting of seeds after they are sown in the soil bacterial life has its influence. When seeds are placed m moist soil they germinate under the influence of heat. The rich alb.u.minous material in the seeds furnishes excellent food, and inasmuch as bacteria abound in the soil, it is inevitable that they should grow in and feed upon the seed. If the moisture is excessive and the heat considerable, they very frequently grow so rapidly in the seed as to destroy its life as a seedling. The seed rots in the ground as a result. This does not commonly occur, however, in ordinary soil. But even here bacteria do grow in the seed, though not so abundantly as to produce any injury. Indeed, it has been claimed that their presence in the seed in small quant.i.ties is a necessity for the proper sprouting of the seed. It has been claimed that their growth tends to soften the food material in the seed, so that the young seedling can more readily absorb it for its own food, and that without such a softening the seed remains too hard for the plant to use. This may well be doubted, however, for seeds can apparently sprout well enough without the aid of bacteria. But, nevertheless, bacteria do grow in the seed during its germination, and thus do aid the plant in the softening of the food material. We can not regard them as essential to seed germination. It may well be claimed that they ordinarily play at least an incidental part in this fundamental life process, although it is uncertain whether the growth of seedlings is to any considerable extent aided thereby.

THE SILO.

In the management of a silo the farmer has undoubtedly another great bacteriological problem. In the attempt to preserve his summer-grown food for the winter use of his animals, he is hindered by the activity of common bacteria. If the food is kept moist, it is sure to undergo decomposition and be ruined in a short time as animal food. The farmer finds it necessary, therefore, to dry some kinds of foods, like hay. While he can thus preserve some foods, others can not be so treated. Much of the rank growth of the farm, like cornstalks, is good food while it is fresh, but is of little value when dried. The farmer has from experience and observation discovered a method of managing bacterial growth which enables him to avoid their ordinary evil effects. This is by the use of the silo. The silo is a large, heavily built box, which is open only at the top. In the silo the green food is packed tightly, and when full all access of air is excluded, except at its surface. Under these conditions the food remains moist, but nevertheless does not undergo its ordinary fermentations and putrefactions, and may be preserved for months without being ruined. The food in such a silo may be taken out months after it is packed, and will still be found to be in good condition for food. It is true that it has changed its character somewhat, but it is not decayed, and is eagerly eaten by cattle.

We are yet very ignorant of the nature of the changes which occur m the food while in the silo. The food is not preserved from fermentation. When the siloxis packed slowly, a very decided fermentation occurs by which the ma.s.s is raised to a high temperature (140 degrees F. to 160 degrees F.). This heating is produced by certain species of bacteria which grow readily even at this high temperature. The fermentation uses up the air in the silo to a certain extent and produces a settling of the material which still further excludes air. The first fermentation soon ceases, and afterward only slow changes occur. Certain acid- producing bacteria after a little begin to grow slowly, and in time the silage is rendered somewhat sour by the production of acetic acid. But the exclusion of air, the close packing, and the small amount of moisture appear to prevent the growth of the common putrefactive bacteria, and the silage remains good for a long time. In other methods of filling the silo, the food is very quickly packed and densely crowded together so as to exclude as much air as possible from the beginning. Under these conditions the lack of moisture and air prevents fermentative action very largely. Only certain acid-producing organisms grow, and these very slowly. The essential result in either case is that the common putrefactive bacteria are prevented from growing, probably by lack of sufficient oxygen and moisture, and thus the decay is prevented. The closely packed food offers just the same unfavourable condition for the growth of common putrefactive bacteria that we have already seen offered by the hard-pressed cheese, and the bacteria growth is in the same way held in check.

Our knowledge of the matter is as yet very slight, but we do know enough to understand that the successful management of a silo is dependent upon the manipulation of bacteria.

THE FERTILITY OF THE SOIL.

The farmer's sole duty is to extract food from the soil. This he does either directly by raising crops, or indirectly by raising animals which feed upon the products of the soil. In either case the fertility of the soil is the fundamental factor in his success. This fertility is a gift to him from the bacteria.

Even in the first formation of soil he is in a measure dependent upon bacteria. Soil, as is well known, is produced in large part by the crumbling of the rocks into powder. This crumbling we generally call weathering, and regard it as due to the effect of moisture and cold upon the rocks, together with the oxidizing action of the air. Doubtless this is true, and the weathering action is largely a physical and chemical one. Nevertheless, in this fundamental process of rock disintegration bacterial action plays a part, though perhaps a small one. Some species of bacteria, as we have seen, can live upon very simple foods, finding in free nitrogen and carbonates sufficiently highly complex material for their life. These organisms appear to grow on the bare surface of rocks, a.s.similating nitrogen from the air, and carbon from some widely diffused carbonates or from the CO2 in the air. Their secreted products of an acid nature help to soften the rocks, and thus aid in performing the first step in weathering.

The soil is not, however, all made up of disintegrated rocks. It contains, besides, various ingredients which combine to make it fertile. Among these are various sulphates which form important parts of plant foods. These sulphates appear to be formed, in part, at least, by bacterial agency. The decomposition of proteids gives rise, among other things, to hydrogen sulphide (H2S). This gas, which is of common occurrence in the atmosphere, is oxidized by bacterial growth into sulphuric acid, and this is the basis of part of the soil sulphates. The deposition of iron phosphates and iron silicates is probably also in a measure aided by bacterial action. All of these processes are factors in the formation of soil. Beyond much question the rock disintegration which occurs everywhere in Nature is chiefly the result of physical and chemical changes, but there is reason for believing that the physical and chemical processes are, to a slight extent at least, a.s.sisted by bacterial life.

A more important factor of soil fertility is its nitrogen content, without which it is completely barren. The origin of these nitrogen ingredients has been more or less of a puzzle. Fertile soil everywhere contains nitrates and other nitrogen compounds, and in certain parts of the world there are large acc.u.mulations of these compounds, like the nitrate beds of Chili. That they have come ultimately from the free atmospheric nitrogen seems certain, and various attempts have been made to explain a method of this nitrogen fixation. It has been suggested that electrical discharges in the air may form nitric acid, which would readily then unite with soil ingredients to form nitrates. There is little reason, however, for believing this to be a very important factor But in the soil bacteria we find undoubtedly an efficient agency m this nitrogen fixation. As already seen, the bacteria are able to seize the free atmospheric nitrogen, converting it into nitrite and nitrates. We have also learned that they can act in connection with legumes and some other plants, enabling them to fix atmospheric nitrogen and store it m their roots. By these two means the nitrogen ingredient in the soil is prevented from becoming exhausted by the processes of dissipation constantly going on. Further, by some such agency must we imagine the original nitrogen soil ingredient to have been derived. Such an organic agency is the only one yet discerned which appears to have been efficient in furnis.h.i.+ng virgin soil with its nitrates, and we must therefore look upon bacteria as essential to the original fertility of the soil. But in another direction still does the farmer depend directly upon bacteria The most important factor in the fertility of the soil is the part of it called humus. This humus is very complex, and never alike in different soils It contains nitrogen compounds in abundance, together with sulphates, phosphates, sugar, and many other substances. It is this which makes the garden soil different from sand, or the rich soil different from the sterile soil. If the soil is cultivated year after year, its food ingredients are slowly but surely exhausted.

Something is taken from the humus each year, and unless this be replaced the soil ceases to be able to support life. To keep up a constant yield from the soil the farmer understands that he must apply fertilizers more or less constantly.

This application of fertilizers is simply feeding the crops. Some of these fertilizers the farmer purchases, and knows little or nothing as to their origin. The most common method of feeding the crops is, however, by the use of ordinary barnyard manure. The reason why this material contains plant food we can understand, since it is made of the undigested part of food, together with all the urea and other excretions of animals, and contains, therefore, besides various minerals, all of the nitrogenous waste of animal life. These secretions are not at first fit for plant food. The farmer has learned by experience that such excretions, before they are of any use on his fields, must undergo a process of slow change, which is sometimes called ripening. Fresh manure is sometimes used on the fields, but it is only made use of by the plants after the ripening process has occurred. Fresh animal excretions are of little or no value as a fertilizer. The farmer, therefore, commonly allows it to remain in heaps for some time, and it undergoes a slow change, which gradually converts it into a condition in which it can be used by plants. This ripening is readily explained by the facts already considered The fresh animal secretions consist of various highly complex compounds of nitrogen, and the ripening is a process of their decomposition.

The proteids are broken to pieces, and their nitrogen elements reduced to the form of nitrates, leucin, etc, or even to ammonia or free nitrogen. Further, a second process occurs, the process of oxidation of these nitrogen compounds already noticed, and the ammonia and nitrites resulting from the decomposition are built into nitrates. In short, in this ripening manure the processes noticed in the first part of this chapter are taking place, by which the complex nitrogenous bodies are first reduced and then oxidized to form plant food. The ripening of manure is both an a.n.a.lytical and a synthetical process. By the a.n.a.lysis, proteids and other bodies are broken into very simple compounds, some of them, indeed, being dissipated into the air, but other portions are retained and then oxidized, and these latter become the real fertilizing materials. Through the agency of bacteria the compost heap thus becomes the great source of plant food to the farmer.

Into this compost heap he throws garbage, straw, vegetable and animal substances in general, or any organic refuse which may be at hand. The various bacteria seize it all, and cause the decomposition which converts it into plant food again. The rotting of the compost heap is thus a gigantic cultivation of bacteria.

This knowledge of the ripening process is further teaching the farmer how to prevent waste. In the ordinary decomposition of the compost heap not an inconsiderable portion of the nitrogen is lost in the air by dissipation as ammonia or free nitrogen. Even his nitrates may be thus lost by bacterial action. This portion is lost to the farmer completely, and he can only hope to replace it either by purchasing nitrates in the form of commercial fertilizers, or by reclaiming it from the air by the use of the bacterial agencies already noticed. With the knowledge now at his command he is learning to prevent this waste. In the decomposition one large factor of loss is the ammonia, which, being a gas, is readily dissipated into the air. Knowing this common result of bacterial action, the scientist has told the farmer that, by adding certain common chemicals to his decomposing manure heap, chemicals which will readily unite with ammonia, he may retain most of the nitrogen in this heap in the form of ammonia salts, which, once formed, no longer show a tendency to dissipate into the air. Ordinary gypsum, or superphosphates, or plaster will readily unite with ammonia, and these added to the manure heap largely counteract the tendency of the nitrogen to waste, thus enabling the farmer to put back into his soil most of the nitrogen which was extracted from it by his crops and then used by his stock. His vegetable crops raise the nitrates into proteids. His animals feed upon the proteids, and perform his work or furnish him with milk. Then his bacteria stock take the excreted or refuse nitrogen, and in his manure heap turn it back again into nitrates ready to begin the circle once more. This might go on almost indefinitely were it not for two facts, the farmer sends nitrogenous material off his farm in the milk or grains or other nitrogenous products, which he sells, and the decomposition processes, as we have seen, dissipate some of the nitrogen into the air as free nitrogen.

To meet this emergency and loss the farmer has another method of enriching the soil, again depending upon bacteria. This is the so- called green manuring. Here certain plants which seize nitrogen from the air are cultivated upon the field to be fertilized, and, instead of harvesting a crop, it is ploughed into the soil. Or perhaps the tops may be harvested, the rest being ploughed into the soil. The vegetable material thus ploughed in lies over a season and enriches the soil. Here the bacteria of the soil come into play in several directions. First, if the crop sowed be a legume, the soil bacteria a.s.sist it to seize the nitrogen from the air. The only plants which are of use in this green manuring are those which can, through the agency of bacteria, obtain nitrogen from the air and store it in their roots. Second, after the crop is ploughed into the soil various decomposing bacteria seize upon it, pulling the compounds to pieces. The carbon is largely dissipated into the air as carbonic dioxide, where the next generation of plants can get hold of it. The minerals and the nitrogen remain in the soil. The nitrogenous portions go through the same series of decomposition and synthetical changes already described, and thus eventually the nitrogen seized from the air by the combined action of the legumes and the bacteria is converted into nitrates, and will serve for food for the next set of plants grown on the same soil. Here is thus a practical method of using the nitrogen a.s.similation powers of bacteria, and reclaiming nitrogen from the air to replace that which has been lost. Thus it is that the farmer's nitrogen problem of the fertile soil appears to resolve itself into a proper handling of bacteria. These organisms have stocked his soil in the first place. They convert all of his compost heap wastes into simple bodies, some of which are changed into plant foods, while others are at the same time lost. Lastly, they may be made to reclaim this lost nitrogen, and the fanner, so soon as he has requisite knowledge of these facts, will be able to keep within his control the supply of this important element. The continued fertility of the soil is thus a gift from the bacteria.

BACTERIA AS SOURCES OF TROUBLE TO THE FARMER.

While the topics already considered comprise the most important factors in agricultural bacteriology, the farmer's relations to bacteria do not end here. These organisms come incidentally into his life in many ways. They are not always his aids as they are in most of the instances thus far cited. They produce disease in his cattle, as will be noticed in the next chapter. Bacteria are agents of decomposition, and they are just as likely to decompose material which the farmer wishes to preserve as they are to decompose material which the farmer desires to undergo the process of decay. They are as ready to attack his fruits and vegetables as to ripen his cream. The skin of fruits and vegetables is a moderately good protection of the interior from the attack of bacteria; but if the skin be broken in any place, bacteria get in and cause decay, and to prevent it the farmer uses a cold cellar.

The bacteria prevent the farmer from preserving meats for any length of time unless he checks their growth in some way. They get into the eggs of his fowls and ruin them. Their troublesome nature in the dairy in preventing the keeping of milk has already been noticed. If he plants his seeds in very moist, damp weather, the soil bacteria cause too rapid a decomposition of the seeds and they rot in the ground instead of sprouting. They produce disagreeable odours, and are the cause of most of the peculiar smells, good and bad, around the barn. They attack the organic matter which gets into his well or brook or pond, decomposing it, filling the water with disagreeable and perhaps poisonous products which render it unfit to drink. They not only aid in the decay of the fallen tree in his forests; but in the same way attack the timber which he wishes to preserve, especially if it is kept in a moist condition. Thus they contribute largely to the gradual destruction of wooden structures. It is therefore the presence of these organisms which forces him to dry his hay, to smoke his hams, to corn his beef, to keep his fruits and vegetables cool and prevent skin bruises, to ice his dairy, to protect his timber from rain, to use stone instead of wooden foundations for buildings, etc. In general, when the farmer desires to get rid of any organic refuse, he depends upon bacteria, for they are his sole agents (aside from fire) for the final destruction of organic matter.

When he wishes to convert waste organic refuse into fertilizing material, he uses the bacteria of his compost heap. On the other hand, whenever he desires to preserve organic material, the bacteria are the enemies against which he must carefully guard.

Thus the farmer's life from year's end to year's end is in most intimate a.s.sociation with bacteria. Upon them he depends to insure the continued fertility of his soil and the constant continued production of good crops. Upon them he depends to turn into plant food all the organic refuse from his house or from his barn. Upon them he depends to replenish his stock of nitrogen. It is these organisms which furnish his dairy with its b.u.t.ter flavours and with the taste of its cheese. But, on the other hand, against them he must be constantly alert. All his food products must be protected from their ravages. A successful farmer's life, then, largely resolves itself into a skilful management of bacterial activity. To aid them in destroying or decomposing everything which he does not desire to preserve, and to prevent their destroying the organic material which he wishes to keep for future use, is the object of a considerable portion of farm labour; and the most successful farmer to-day, and we believe the most successful farmer of the future, is the one who most intelligently and skilfully manipulates these gigantic forces furnished him by the growth of his microscopical allies.

RELATION OF BACTERIA TO COAL. Another one of Nature's processes in which bacteria have played an important part is in the formation of coal. It is unnecessary to emphasize the importance of coal in modern civilization. Aside from its use as fuel, upon which civilization is dependent, coal is a source of an endless variety of valuable products. It is the source of our illuminating gas, and ammonia is one of the products of the gas manufacture. From the coal also comes coal tar, the material from which such a long series of valuable materials, as aniline colours, carbolic acid, etc, is derived. The list of products which we owe to coal is very long, and the value of this material is hardly to be overrated. In the preparation of these ingredients from coal bacteria do not play any part. Most of them are derived by means of distillation.

But when asked for the agents which have given us the coal of the coal beds, we shall find that here, too, we owe a great debt to bacteria.

Coal, as is well known, has come from the acc.u.mulation of the luxuriant vegetable growth of the past geological ages. It has therefore been directly furnished us by the vegetation of the green plants of the past, and, in general, it represents so much carbonic dioxide which these plants have extracted from the atmosphere. But while the green plants have been the active agents in producing this a.s.similation, bacteria have played an important part in coal manufacture in two different directions. The first appears to be in furnis.h.i.+ng these plants with nitrogen. Without a store of fixed nitrogen in the soil these carboniferous plants could not have grown. This matter has already been considered. We have no very absolute knowledge as to the agency of bacteria in furnis.h.i.+ng nitrogen for this vegetation in past ages, but there is every reason to believe that in the past, as in the present, the chief source of organic nitrogen has been from the atmosphere and derived from the atmosphere through the agency of bacteria. In the absence of any other known factor we may be pretty safe in the a.s.sumption that bacteria played an important part in this nitrogen fixation, and that bacteria must therefore be regarded as the agents which have furnished us the nitrogen stored in the coal.

But in a later stage of coal formation bacteria have contributed more directly to the formation of coal. Coal is not simply acc.u.mulated vegetation. The coal of our coal beds is very different in its chemical composition from the wood of the trees.

It contains a much higher percentage of carbon and a lower percentage of hydrogen and oxygen than ordinary vegetable substances. The conversion of the vegetation of the carboniferous ages into coal was accompanied by a gradual loss of hydrogen and a consequent increase in the percentage of carbon. It is this change that has added to the density of the substance and makes the greater value of coal as fuel. There is little doubt now as to the method by which this woody material of the past has been converted into coal. The same process appears to be going on in a similar manner to-day in the peat beds of various northern countries. The fallen vegetation, trees, trunks, branches, and leaves, acc.u.mulate in ma.s.ses, and, when the conditions of moisture and temperature are right, begin to undergo a fermentation. Ordinarily this action of bacteria, as already noticed, produces an almost complete though slow oxidation of the carbon, and results in the total decay of the vegetable matter. But if the vegetable ma.s.s be covered by water and mud under proper conditions of moisture and temperature, a different kind of fermentation arises which does not produce such complete decay. The covering of water prevents the access of oxygen to the fermenting ma.s.s, an oxidation of the carbon is largely prevented, and the vegetable matter slowly changes its character. Under the influence of this slow fermentation, aided, probably by pressure, the ma.s.s becomes more and more solid and condensed, its woody character becomes less and less distinct, and there is a gradual loss of the hydrogen and the oxygen. Doubtless there is a loss of carbon also, for there is an evolution of marsh gas which contains carbon. But, in this slow fermentation taking place under the water in peat bogs and marshes the carbon loss is relatively small; the woody material does not become completely oxidized, as it does in free operations of decay. The loss of hydrogen and oxygen from the ma.s.s is greater than that of carbon, and the percentage of carbon therefore increases. This is not the ordinary kind of fermentation that goes on in vegetable acc.u.mulations. It requires special conditions and possibly special kinds of fermenting organisms. Peat is not formed in all climates. In warm regions, or where the woody matter is freely exposed to the air, the fermentation of vegetable matter is more complete, and it is entirely destroyed by oxidation. It is only in colder regions and when covered with water that the destruction of the organic matter stops short of decay. But such incomplete fermentation is still going on in many parts of the world, and by its means vegetable acc.u.mulations are being converted into peat.

This formation of peat appears to be a first step in the formation of denser coal. By a continuation of the same processes the ma.s.s becomes still more dense and solid. As we pa.s.s from the top to the bottom of such an acc.u.mulation of peat, we find it becoming denser and denser, and at the bottom it is commonly of a hard consistence, brownish in colour, and with only slight traces of the original woody structure. Such material is called lignite. It contains a higher percentage of carbon than peat, but a lower percentage than coal, and is plainly a step in coal formation. But the process goes on, the hydrogen and oxygen loss continuing until there is finally produced true coal.

If this is the correct understanding of the formation of coal, we see that we have plainly a process in which bacterial life has had a large and important share. We are, of course, densely ignorant of the exact processes going on. We know nothing positively as to the kind of microorganisms which produce this slow, peculiar fermentation. As yet, the fermentation going on in the formation of the peat has not been studied by the bacteriologists, and we do not know from direct experiment that it is a matter of bacterial action. It has been commonly regarded as simply a slow chemical change, but its general similarity to other fermentative processes is so great that we can have little hesitation in attributing it to micro-organisms, and doubtless to some forms of plants allied to bacteria. There is no reason for doubting that bacteria existed in the geological ages with essentially the same powers as they now possess, and to some forms of bacteria which grow in the absence of oxygen can we probably attribute the slow change which has produced coal. Here, then, is another great source of wealth in Nature for which we are dependent upon bacteria. While, of course, water and pressure were very essential factors in the deposition of coal, it was a peculiar kind of fermentation occurring in the vegetation that brought about the chemical changes in it which resulted in its transformation into coal. The vegetation of the carboniferous age was dependent upon the nitrogen fixed by the bacteria, and to these organisms also do we owe the fact that this vegetation was stored for us in the rocks.

CHAPTER V.

PARASITIC BACTERIA AND THEIR RELATION TO DISEASE.

Perhaps the most universally known fact in regard to bacteria is that they are the cause of disease. It is this fact that has made them objects of such wide interest. This is the side of the subject that first attracted attention, has been most studied, and in regard to which there has been the greatest acc.u.mulation of evidence. So persistently has the relation of bacteria to disease been discussed and emphasized that the majority of readers are hardly able to disa.s.sociate the two. To most people the very word bacteria is almost equivalent to disease, and the thought of swallowing microbes in drinking water or milk is decidedly repugnant and alarming. In the public mind it is only necessary to demonstrate that an article holds bacteria to throw it under condemnation.

We have already seen that bacteria are to be regarded as agents for good, and that from their fundamental relation to plant life they must be looked upon as our friends rather than as our enemies. It is true that there is another side to the story which relates to the parasitic species. These parasitic forms may do us direct or indirect injury. But the species of bacteria which are capable of doing us any injury, the pathogenic bacteria, are really very few compared to the great host of species which are harmless. A small number of species, perhaps a score or two, are pathogenic, while a much larger number, amounting to hundreds and perhaps thousands of species, are perfectly harmless. This latter cla.s.s do no injury even though swallowed by man in thousands. They are not parasitic, and are unable to grow in the body of man.

Their presence is entirely consistent with the most perfect health, and, indeed, there are some reasons for believing that they are sometimes directly beneficial to health. It is entirely unjust to condemn all bacteria because a few chance to produce mischief. Bacteria in general are agents for good rather than ill.

There are, however, some species which cause mankind much trouble by interfering in one way or another with the normal processes of life. These pathogenic bacteria, or disease germs, do not all act alike, but bring about injury to man in a number of different ways. We may recognise two different cla.s.ses among them, which, however, we shall see are connected by intermediate types. These two cla.s.ses are, first, the pathogenic bacteria, which are not strictly parasitic but live free in Nature; and, second, those which live as true parasites in the bodies of man or other animals. To understand the real relation of these two cla.s.ses, we must first notice the method by which bacteria in general produce disease.

METHOD BY WHICH BACTERIA PRODUCE DISEASE.

Since it was first clearly recognised that certain species of bacteria have the power of producing disease, the question as to how they do so has ever been a prominent one Even if they do grow in the body, why should their presence give rise to the symptoms characterizing disease? Various answers to this question have been given in the past It has been suggested that in their growth they consume the food of the body and thus exhaust it, that they produce an oxidation of the body tissues, or that they produce a reduction of these tissues, or that they mechanically interfere with the circulation None of these suggestions have proved of much value Another view was early advanced, and has stood the test of time. This claim is that the bacteria while growing in the body produce poisons, and these poisons then have a direct action on the body We have already noticed that bacteria during their growth in any medium produce a large number of biproducts of decomposition. We noticed also that among these biproducts there are some which have a poisonous nature; so poisonous are they that when inoculated into the body of an animal they may produce poisoning and death. We have only to suppose that the pathogenic bacteria, when growing as parasites in man, produce such poisons, and we have at once an explanation of the method by which they give rise to disease.

This explanation of germ disease is more than simple theory. It has been in many cases clearly demonstrated. It has been found that the bacteria which cause diphtheria, teta.n.u.s, typhoid, tuberculosis, and many other diseases, produce, even when growing in common culture media, poisons which are of a very violent nature. These poisons when inoculated into the bodies of animals give rise to much the same symptoms as the bacteria do themselves when growing as parasites in the animals. The chief difference in the results from inoculating an animal with the poison and with the living bacteria is in the rapidity of the action. When the poison is injected the poisoning symptoms are almost immediately seen, but when the living bacteria are inoculated the effect is only seen after several days or longer, not, in short, until the inoculated bacteria have had time enough to grow in the body and produce the poison in quant.i.ty. It has not by any means been shown that all pathogenic germs produce their effect in this way, but it has been proved to be the real method in quite a number of cases, and is extremely probable in others. While some bacteria perhaps produce results by a different method, we must recognise the production of poisons as at all events the common direct cause of the symptoms of disease. This explanation will enable us more clearly to understand the relation of different bacteria to disease.

PATHOGENIC GERMS WHICH ARE NOT STRICTLY PARASITIC

Recognising that bacteria may produce poisons, we readily see that it is not always necessary that they should be parasites in order to produce trouble. In their ordinary growth in Nature such bacteria will produce no trouble The poisons will be produced in decaying material but will seldom be taken into the human body.