A Catechism of Familiar Things Part 49

Not a particle; the story of a stone having the property of converting the baser metals into gold being merely an absurd fable: yet, although the pursuits of Alchemy were the most preposterous that can be conceived, the ardor with which they were followed, and the amazing number of experiments made in consequence, led to the discovery of many facts to which Chemistry is highly indebted.

_Preposterous_, absurd, foolish; contrary to nature or reason.

You inform me that Chemistry enables us to discover the properties of bodies by means of _a.n.a.lysis_ and _combination_: what do these terms imply?

If a chemist wishes to examine the properties of a compound body, he proceeds by a.n.a.lysis--that is, by a separation of the substance to be examined into its const.i.tuent parts. The chemical examination of bodies is generally effected by producing a change in the _nature_ or _state_ of the body under examination. This change is frequently brought about by the addition of some _other_ substance which forms a combination with a part of the substance examined, and leaves the remainder in a detached state.

By what _means_ do Chemists effect a change in the qualities or states of natural bodies?

It is generally effected by means of _heat_, which has a tendency to separate the particles of bodies from each other; or by the _mixture_ or _combination_ of some other matter with the matter intended to be examined. The mixture of two or more compounds often produces a decomposition by means of chemical _affinity_, a property which different species of matter have to unite with each other; and which is sometimes called _elective affinity_. Thus it may be observed, chemists have not only the power of decomposing natural bodies, but of producing by combination various other substances, such as are not found in the kingdom of nature.

What do you mean by _decomposition_?

In chemical language, it means the separation of a compound body into its simple elements.

Give me an example.

Water may be decomposed, and reduced into oxygen and hydrogen,--both of them simple substances incapable of further decomposition.

Is not the work of decomposition perpetually going forward?

Yes; and _combustion_ is one of the great agents in this work. By it animal and vegetable substances are converted into water and carbonic acid, by the union of their hydrogen and carbon with the oxygen of the air. These, in time, are again absorbed by vegetables, and again decomposed to set the oxygen at liberty to produce fresh combustions.

Of what use are the two remaining substances, Hydrogen and Carbon?

These are appropriated by the vegetative organs to their growth and nourishment, while the oxygen with which the carbon was combined is abundantly given off to purify the air and render it fit for the respiration of animals.

Give me an idea of the mode in which Chemists ascertain the _affinity_ of bodies, by relating an experiment.

Dissolve a tea-spoonful of sugar of lead in water, and pour the clear solution into a decanter or large gla.s.s bottle. Then take a small piece of zinc, and twist round it some bra.s.s or copper wire, so as to let the ends of the wire depend from it in any agreeable form. Suspend the zinc and wire in the solution which has been prepared; in a short time, metallic lead will deposit itself on the zinc and along the wire. This is a beautiful ill.u.s.tration of chemical affinity; the acid, which const.i.tutes a part of the sugar of lead, has a stronger affinity for the zinc than for the lead, and, consequently, will combine with the zinc, and form a compound which remains in solution, while the lead is precipitated on the zinc and wire in the form of a brilliant tree of metal.

_Affinity_, in chemistry, that attraction which takes place between the elements of bodies, and forms compounds.

What does the word Nature signify?

In the above sense, the system of the universe; the creation, the works of G.o.d. By the kingdom of nature is meant the world and all things in it: nature is divided into three kingdoms, the animal, vegetable, and mineral.

What are the different states of natural bodies?

All bodies are either solid, liquid, or aeriform. By solid bodies are meant those whose parts unite so firmly as to resist the impression or penetration of other bodies; by liquid, those substances whose parts do not unite firmly, but have free motion among themselves; by aeriform, fluid substances, having the form or nature of air. Liquid substances are nothing more than solids converted into liquids by heat, a certain increase of which would convert the liquids into vapor.

What other name is given to Liquids?

They are likewise called fluids: we call the air, also, a fluid, because it flows like a fluid, and light substances will float in it.

What is the cause of bodies floating on liquids?

It is an established law of nature, that all substances which weigh less than an equal bulk of any liquid, will float on the surface of this liquid. Thus a cork will float on water, while a stone sinks to the bottom. The cork will not float in the air, though lighter than water; and the stone is not heavier than the _whole_ of the water, but more so than a portion of water of its _own bulk_,--and thus it sinks in it. Stones also differ in their weight or gravity: for instance, some of the asbestus kind are _lighter_ than water. Iron, bra.s.s, indeed, nearly all substances, except gold and platina, will float upon mercury, because they are lighter than this liquid.

What is the cause of bodies being either solid, liquid, or aeriform?

When the principle of _attraction_ prevails, it causes them to become solid; when caloric prevails, they become aeriform. Fluidity is, apparently, a medium between the two.

How is the state of Solidity in bodies accounted for?

The particles of all bodies are subject to two opposite powers, _repulsion_ and _attraction_; between which they remain in equilibrium. While the _attractive_ force remains strongest, the body remains in a state of solidity; but if heat destroys this force, the particles lose their cohesion, and the body ceases to be solid.

_Cohesion_, act of sticking together, union of the const.i.tuent parts of a body.

Which is supposed to be the most natural state of all bodies?

Solidity; for by the _combination_ of caloric with them we can reduce most substances to the fluid state; while the greatest number of _liquid_ substances take a _solid_ form by the loss of caloric. Thus, water congeals and forms ice; and even the gases show this disposition to become solid, when they lose their _elasticity_ by forming some _combination_.

Explain the terms _Repulsion_ and _Attraction_.

Repulsion is a peculiar property in the particles of matter, which gives them a constant tendency to recede from each other. Attraction is an unknown force, which causes bodies or their particles to approach each other. The particles of all bodies possess this property, which causes them to adhere, and preserves the various substances around us from falling in pieces.

What different kinds of Attraction can you mention?

Attraction may be distinguished into that which takes place between bodies at sensible distances, and that which manifests itself between the _particles_ of matter at insensible distances.

Give an example of the first kind of attraction.

One of the most familiar instances of attraction at sensible distances is seen in the descent of heavy bodies to the ground. When a stone is lifted up in the hand, the earth's attraction, which previously caused it to remain at its surface, is overcome; but, as soon as the hand is withdrawn, the stone falls to the earth. The force which causes this is called the _attraction of gravitation_, or simply _gravitation_.

How is the second kind of attraction, or that between the particles of bodies, subdivided?

Into the _attraction of aggregation_, or _cohesion_; and _chemical attraction_, or _affinity_. The former takes place between particles which are _similar_, and the latter between those which are _dissimilar_. All the operations of chemistry are founded upon the force of affinity which Nature has established between the particles of different kinds of matter, and which enables the chemist to produce _new_ compounds differing more or less from the substances by whose union they were formed.

Is it, then, necessary for chemists to understand the relative nature of all substances?