Heads of Lectures on a Course of Experimental Philosophy: Particularly Including Chemistry Part 8

_Of Siliceous Earth._

Siliceous earth seems to be formed by nature from chalk, perhaps by the introduction of some unknown acid, which the vitriolic acid is not able to dislodge. It abounds in most substances which are hard enough to strike fire with steel, as _flint_, _rock crystal_, and most _precious stones_. It is not acted upon by any acid except the fluor and phosphoric, but especially the former: but it is soluble in alkalies; and being then dissolved in water, makes _liquor silic.u.m_, from which the purest siliceous earth may be precipitated by acids. For this purpose about four times the weight of alkali must be made use of. With about equal weights of alkali and siliceous sand is made _gla.s.s_, of so great use in admitting light and excluding the weather from our houses, as well as for making various useful utensils. To make gla.s.s perfectly colourless, and at the same time more dense, commonly called _flint gla.s.s_, manufacturers use a certain proportion of calx of lead and manganese.

Siliceous earth is not affected by the strongest heat, except by means of a burning lens, or dephlogisticated air.

LECTURE XX.

_Of Argillaceous Earth._

Argillaceous earth is found in _clay_, _schistus_, or _slate_, and in _mica_; but the purest is that which is precipitated from a solution of alum by alkalies; for alum consists of the union of vitriolic acid and argillaceous earth.

This species of earth is ductile with water; it then hardens and contracts by heat, so as to be of the greatest use in forming _bricks_, or stones of any required form or size. By means of the property of clay to contract in the fire, Mr. Wedgwood has constructed an excellent thermometer to measure the degrees of extreme heat.

The ductility of clay seems to depend upon some acid, probably the vitriolic, adhering to it; for it loses that property when it is burned into a brick, but recovers it when it has been again dissolved in an acid.

_Of Terra Ponderosa._

_Terra ponderosa_, or _marmor metallic.u.m_, is generally found in two states, viz. united to vitriolic acid, when it is called _calk_, or to fixed air, when it is called _terra ponderosa aerata_.

To obtain it pure from its union with the vitriolic acid, it must be melted with about twice its weight of fixed alkali; which unites with the acid, and forming a saline substance, may be washed out of it. In this state it contains water, and therefore, when exposed to heat, will yield fixed air; whereas the terra ponderosa aerata will not yield fixed air by heat only, but when steam is made to pa.s.s over it when red hot.

This proves that water is essential to the composition of fixed air.

This stone is distinguishable by its great specific gravity, being four times as heavy as water; but though in this it resembles an _ore_, it has not been found to be metallizable.

_Of Magnesia._

This species of earth is found in _steat.i.tes_, or _soap rock_, _Spanish chalk_, _asbestus_, and _Muscovy talck_; but the purest is got by dissolving _Epsom salts_ (which consists of this earth united to the vitriolic acid) and precipitating it by a mild alkali. In this state it becomes united to fixed air, which may be expelled by heat. It is then _calcined_, or _caustic_, but differs from quick-lime by not being soluble in water.

_Asbestus_, which contains much of this kind of earth, is remarkable for not being destructible by heat, though it is sometimes found in flexible fibres, so as to be capable of being woven into cloth.

_Muscovy talck_ is remarkable for the thin and transparent flakes into which it is divisible, and thereby capable of various uses.

There are some other distinct species of earth, particularly one brought from Botany Bay, and another called _Stontiate_, from the place where it was found in Scotland; but they have not as yet been much examined.

All stones formed by nature are compounded, and to distinguish them from one another, and ascertain the parts of which they consist, is the subject of _lithology_, a very extensive branch of knowledge.

All the simple earths are nearly, if not absolutely, _infusible_; but when they are mixed they may all be fused.

LECTURE XXI.

_Of Ores_.

Metallizable earths, commonly called _ores_, when united to phlogiston, make the metals, distinguishable for their specific gravity, their opacity, shining appearance, and fusibility.

All the proper metals are _malleable_, and those which are not so are called _semi-metals_.

The metals again are subdivided into the _perfect_ and _imperfect_. The former, which are _gold_, _silver_, and _platina_, suffer no change by fusion, or the longest continued heat: whereas heat calcines or dissipates the phlogiston of the imperfect metals, which are _mercury_, _lead_, _copper_, _iron_, and _tin_, so that they return to the state of earth; and this earth is always heavier than the metal, though of less specific gravity, having received an addition of weight from water or air: but these earths, or ores, being exposed to heat in contact with substances containing phlogiston, again become metals, and are then said to be _revived_.

The semi-metals are _bis.m.u.th_, _zinc_, _nickel_, regulus of _a.r.s.enic_, of _cobalt_, of _antimony_, of _manganese_, of _wolfram_, and of _molybdena_.

All metallic substances are crystallizable, and each in a peculiar form, which is discovered by leaving a hole in the bottom of the crucible in which they are melted, and drawing out the stopper, when the ma.s.s is beginning to lose its fluidity.

Some of the metals will not unite to others when hot, and others of them will; and such as will unite with others are called _solders_. Thus tin is a solder for lead, and bra.s.s, gold, or silver, for iron.

Ores are never found in regular strata, like the different kinds of earth; but in places which have formerly been cavities, running in all directions, with respect to the regular strata, and commonly called _veins_.

Many of the ores in their natural state are said to be _mineralized_ with a.r.s.enic or sulphur, those substances being intimately united with the metallic earths.

In order to convert the ores into metals, some of them are first reduced to powder, to wash out the earthy or saline particles. They are then kept in a red heat, which the workmen call _roasting_, in order to drive away the a.r.s.enic, or sulphur, which are volatile; and in the last place they are fused in contact with charcoal, or other substances containing phlogiston; and to promote the fusion, lime-stone is frequently mixed with them. When the operation is completed, the unmetallic parts are converted into gla.s.s, or _scoria_, which lies on the surface, whereas the metal is found at the bottom.

To discover the quant.i.ty of metal in a small piece of ore is called _a.s.saying_.

When metals are fused together, the specific gravity, fusibility, and other properties are changed, and in such a manner as could not be discovered from the properties of the const.i.tuent parts.

_Of Gold._

Gold is the heaviest of all metallic bodies except platina. It appears yellow or reddish by reflected light, but green or blue by transmitted light, when it is reduced to thin plates.

Though gold undergoes no change in a common furnace, or burning lens, it may, in part, at least, be calcined by the electric shock.

Gold has the greatest _ductility_, and in wires of equal diameters, it has the greatest _tenacity_, of all the metals. One grain of it may be made to cover 56 square inches; some gold leaf being less than a 200,000th part of an inch thick; and when it is made to cover a silver wire, the gold upon it may not be more than one twelfth part of the thickness of the gold leaf.

This metal is soluble in aqua regia; and being precipitated by a volatile alkali, makes a powder called _aurum fulminans_, which is one fourth heavier than the gold, and explodes with great violence in a heat something greater than that of boiling water.

Tin precipitates gold in the form of a purple powder, called the _powder of Ca.s.sius_, from the inventor of it, and is used in enamels, or the gla.s.sy coating which is given to metals by heat.

Gold unites with most of the metals, especially with mercury, and these mixtures are called _amalgams_. In gilding, the amalgam is applied to the surface of the metal to be gilded, and the mercury is driven off by heat, leaving the gold attached to the surface.

Gold mixed with iron, makes it harder, for the purpose of cutting instruments.

To separate gold from the imperfect metals, such as copper, &c. it is mixed with lead, and then exposed to a strong heat, which calcines the lead, and with it the imperfect metals, leaving the gold pure. This process is called _cupellation_, from being performed in a small crucible called a _cupell_. When the gold is mixed with silver, three parts more of silver are put to it, and then the silver is dissolved by nitrous acid, leaving the gold pure. This process is called _quartation_, from the gold being one fourth part of the ma.s.s.

The fineness of gold is generally estimated by dividing the gold into twenty-four parts, called _carats_. The phrase twenty-three carats fine means that the ma.s.s contains twenty-three parts out of twenty-four of pure gold, the remainder being _alloy_, of some baser metal. The fineness of gold may in some measure be discovered by the colour it leaves upon a _touch-stone_, or fine-grained basaltes.

Gold is generally found nearly pure, but mixed with earth, or diffused in fine grains through stones.