Heads of Lectures on a Course of Experimental Philosophy: Particularly Including Chemistry Part 9

LECTURE XXII.

_Of Silver._

Silver is the whitest of all the metals, very ductile, but less so than gold; the thinnest leaves of it being one third thicker than those of gold. It is not calcined in the heat of a common furnace, but partially so by repeated fusion, or a strong burning lens.

Sulphureous fumes unite with silver, and tinge it black. The nitrous acid dissolves it, and will hold more than half its weight of it in solution. When fully saturated, this solution deposits crystals, which are called _lunar nitre_, or _nitre of silver_. When these crystals are melted, and the water they contain driven off, a black substance, called _lapis infernalis_, or _lunar caustic_, is formed. This is used as a cautery in surgery. A strong heat will decompose this lunar nitre, and recover the silver.

Though the nitrous acid dissolves silver the most readily, the marine acid will deprive the nitrous of it, and form a substance called _luna cornea_, because, when it is melted and cold, it becomes a transparent ma.s.s something resembling _horn_. From this luna cornea the purest silver may be obtained. The vitriolic acid will likewise deprive the nitrous of the silver contained in it, and form a white powder, not easily soluble in water.

A fulminating silver may be made by the following process: the silver must first be dissolved in pale nitrous acid, then precipitated by lime-water, dried, and exposed to the air three days. It must then be washed in caustic volatile alkali, after which the fluid must be decanted, and the black powder left to dry in the air. The slightest friction will cause this powder to fulminate. It is said, that even a drop of water falling upon it will produce this effect; so that it ought to be made only in very small quant.i.ties, and managed with the greatest caution.

Most of the metals precipitate silver. That by mercury may be made to a.s.sume the form of a tree, called _arbor Dianae_.

Silver is found native in Peru; and the ores frequently contain sulphur, or a.r.s.enic, or both.

_Of Platina._

Platina is a metal lately discovered in the gold mines of Mexico, where it is found in small particles, never exceeding the size of a pea, mixed with ferruginous sand and quartz.

The strongest fire will not melt these grains, though it will make them cohere; but they may be melted by a burning lens, or a blow-pipe supplied with dephlogisticated air.

Pure platina is the heaviest body in nature, its specific gravity exceeding twenty-two. It is very malleable, though considerably harder than gold or silver, and has the property of welding in common with iron. This metal is not affected by exposure to the air, or by any simple acid, though concentrated and hot; but it is dissolved by dephlogisticated marine acid, and by aqua regia, in which a little nitrous air is procured. The solution is brown, and when diluted yellow.

This liquor is very corrosive, and tinges animal substances of a blackish brown colour. Platina is precipitated from a solution in aqua regia by sal-ammoniac, as gold is by martial vitriol. Iron is precipitated from this solution by the Prussian alkali. Also most of the metals precipitate platina, but not in its metallic state.

a.r.s.enic facilitates the solution of platina; and by melting it with equal parts of a.r.s.enic and vegetable alkali, and then reducing the ma.s.s to a powder, it may be made to take any form; and a strong heat will dissipate the a.r.s.enic and the alkali, leaving the platina in the shape required, not fusible by any heat in a common furnace.

Platina does not readily combine with gold or silver, and it resists the action of mercury as much as iron; but it mixes well with lead, making it less ductile, and even brittle, according to the proportion of the platina. With copper it forms a compound which takes a beautiful polish, not liable to tarnish, and is therefore used with advantage for mirrors of reflecting telescopes. It unites easily with tin, and also with bis.m.u.th, antimony, and zinc.

LECTURE XXIII.

_Of Mercury._

Mercury is the most fusible of all the metals, not becoming solid but in 40 below 0 in Fahrenheit's thermometer. It is then malleable. It is heavier than any other metal except gold or platina. It is volatile in a temperature much lower than that of boiling water, and in vacuo in the common temperature of the atmosphere; and at six hundred it boils.

In a degree of heat in which it would rise easily in vapour, mercury imbibes pure air, and becomes a red calx, called _precipitate per se_.

At a greater degree of heat it parts with that air, and is running mercury again.

Mercury is not perceptibly altered by exposure to the air.

Mercury is acted upon by the vitriolic acid when hot. In this process vitriolic acid air is procured, and the mercury is converted into a white substance, which being dipped in water becomes yellow, called _turbith mineral_, one third heavier than the mercury from which it was made. By heat this substance parts with its pure air, and becomes running mercury; but if the process be made in a clean earthen vessel, there will remain a portion of _red calx_, which cannot be reduced by any degree of heat, except in contact with some substance containing phlogiston. If this be done with a burning lens, in inflammable air, much of the air will be absorbed.

Mercury is dissolved most readily in the nitrous acid, when the purest nitrous air is procured; and there remains a substance which is first yellow, and by continuance red, called _red precipitate_. In a greater degree of heat the dephlogisticated air will be recovered, and the mercury be revived; but the substance yields nitrous air after it becomes solid, and till it changes from yellow to red.

The precipitates of mercury from acids by means of alkalies possess the property of exploding, when they are exposed to a gradual heat in an iron spoon, after having been triturated with one sixth of their weight of the flowers of sulphur. The residuum consists of a violet-coloured powder, which, by sublimation, is converted into cinnabar.

It seems, therefore, as if the sulphur combined suddenly with the mercury, and expelled the dephlogisticated air in an elastic state.

The marine acid seizes upon mercury dissolved in nitrous acid, and if the acid be dephlogisticated, the precipitate is _corrosive sublimate_; but with common marine acid, it is called _calomel_, or _mercurius dulcis_. This preparation is generally made in the dry way, by triturating equal parts of mercury, common salt and vitriol, and exposing the whole to a moderate heat; when the corrosive sublimate rises, and adheres to the upper part of the gla.s.s vessel in which the process is made.

Mercury combines readily with sulphur by trituration, and with it forms a black powder called _Ethiops mineral_. A more intimate combination of mercury and sulphur is made by means of fire. This is called _cinnabar_, about one third of which is sulphur. Vermillion is cinnabar reduced to powder.

Mercury readily unites with oil, and with it forms a deep black or blue compound, used in medicine.

It readily combines with most of the metals, and when it is used in a sufficient quant.i.ty to make them soft, the mixture is called an _amalgam_. It combines most readily with gold, silver, lead, tin, bis.m.u.th, and zinc. Looking-gla.s.ses are covered on the back with an amalgam of mercury and tin.

When mercury is united with lead or other metals, it is rendered less brilliant and less fluid; but agitation in pure air converts the impure metal into a calx, together with much of the mercury, in the form of a black powder.

Heat recovers the pure air, and the mercury, leaving the calx of the impure metal.

Much mercury is found native in a slaty kind of earth, or in ma.s.ses of clay or stone; but the greatest quant.i.ty is found combined with sulphur in _native cinnabar_.

LECTURE XXIV.

_Of Lead._

Lead is a metal of a bluish tinge, of no great tenacity, but very considerable specific gravity, being heavier than silver. It melts long before it is red hot, and is then calcined, if it be in contact with respirable air. When boiling it emits fumes, and calcines very rapidly.

It may be granulated by being poured into a wooden box, and agitated.

During congelation it is brittle, so that the parts will separate by the stroke of a hammer; and by this means the form of its crystals may be discovered.

In the progress of calcination it first becomes a dusky grey powder, then yellow, when it is called _ma.s.sicot_; then, by imbibing pure air, it becomes red, and is called _minium_, or _red lead_. In a greater degree of heat it becomes ma.s.sicot again, having parted with its pure air. If the heat be too great, and applied rapidly, it becomes a flaky substance, called _litharge_; and with more heat it becomes a _gla.s.s_, which readily unites with metallic calces and earths, and is a princ.i.p.al ingredient in the manufacture of _flint gla.s.s_, giving it its peculiar density and refractive power.

Though lead soon tarnishes, the imperfect calx thus made does not separate from the rest of the metal, and therefore protects it from any farther action of the air, by which means it is very useful for the covering of houses, and other similar purposes. All acids act upon lead, and form with it different saline substances. _White-lead_ consists of its union with vinegar and pure air. Also dissolved in vinegar, and crystallized, it becomes _sugar of lead_, which, like all the other preparations of this metal, is a deadly poison.

Oils dissolve the calces of lead, which, by this means, is the basis of paints, plaisters, &c.

By means of heat litharge decomposes common salt, the lead uniting with the marine acid, and forming a yellow substance, used in painting, and by this means the fossil alkali is separated.

Lead unites with most metals, though not with iron. Two parts of lead and one of tin make a _solder_, which melts with less heat than either of the metals separately; but a composition of eight parts of bis.m.u.th, five of lead, and three of tin, makes a metal which melts in boiling water.

This metal will be dissolved by water if it contain any saline matter, and the drinking of it occasions a peculiar kind of cholic.

Lead is sometimes found native, but generally minerally mineralized with sulphur or a.r.s.enic, and often mixed with a small quant.i.ty of silver.

_Of Copper._

Copper is a metal of a reddish or brownish colour, considerably sonorous, and very malleable.