Heads of Lectures on a Course of Experimental Philosophy: Particularly Including Chemistry Part 5

AND FIRST OF

_WATER_.

Having considered all the substances that are usually found in the form of _air_, I come to those that are generally in a _fluid_ form, beginning with _water_, which is the princ.i.p.al, if not the only cause of fluidity to all the other substances that I shall place in this cla.s.s.

Pure water is a liquid substance, transparent, without colour, taste, or smell; and with different degrees of heat and cold may be made to a.s.sume the three forms of a solid, of a fluid, and of air. Below 32 of Fahrenheit it is ice, and above 212 it is vapour; so that in an atmosphere below 32 it never could have been known to be any thing else than a peculiar kind of stone, and above 212 a peculiar species of air.

In pa.s.sing from the state of a solid to that of a liquid, water absorbs a great quant.i.ty of the principle, or matter, of _heat_, which remains in it in a _latent_ state; and in pa.s.sing from a state of fluid to that of vapour, it absorbs much more; and this heat is found when the processes are reversed. It has been observed, that when water becomes vapour, it takes the form of small globules, hollow within, so as to be specifically lighter than air.

The degree of heat at which water is converted into vapour depends upon the pressure of the atmosphere; so that in vacuo, or on the top of a high mountain, it boils with little heat; and when compressed, as in Papin's digester, or in the bottom of a deep pit, it requires much heat.

In the former case the restoring of the pressure will instantly put a stop to the boiling, and in the latter case the removing of the pressure will instantly convert the heated water into vapour.

The ease with which water is converted into vapour by heat, has given a great power to mechanicians, either by employing the natural pressure of the atmosphere, when steam is condensed under a moveable pistern, in an iron cylinder, which was the principle of the old fire-engine, or by employing the elastic power of steam to produce the same effect, which is the principle of Mr. Watt's steam engine.

Water was long thought to be incompressible by any external force, but Mr. Canton has shewn that even the pressure of the atmosphere will condense it very sensibly.

We do not know any external force equal to that by which water is expanded when it is converted into ice, or into vapour. For though the particles of water approach nearer by cold, yet when it crystallizes, the particles arrange themselves in a particular manner, with interstices between them; so that, on the whole, it takes up more room than before.

Water has an affinity to, and combines with, almost all natural substances, aerial, fluid, or solid; but most intimately with acids, alkalies, calcareous earth, and that calx of iron which is called _finery cinder_, from which the strongest heat will not expel it.

It has been supposed by some, that by frequent distillation, and also by agitation, water may be converted into a kind of earth; but this does not appear to be the case. It has also of late been thought, that water is resolvable into dephlogisticated and inflammable air; but the experiments which have been alleged to prove this do not satisfy me; so that, for any thing that appeared till very lately, water might be considered as a simple element. By means of heat, however, it seems to be resolvable into such air as that of which the atmosphere consists, viz. dephlogisticated and phlogisticated, only with a greater proportion of the former.

Water, with respect to specific gravity and temperature, has generally been made the standard to all other substances; its freezing and boiling points being the limits by means of which thermometers are graduated.

Other substances have also been compared with water, as a standard, with respect to the capacity of receiving heat, and retaining it in a latent state, as will be shewn when we consider the subject of heat.

LECTURE XII.

_Of the Nitrous Acid._

Under the head of _liquids_ I shall consider _acids_ and _alkalis_, though some of them may be exhibited in the form of air, and others in a solid form. These two chemical principles are formed to unite with one another, and then they const.i.tute what is called a _neutral salt_.

Both acids and alkalis are distinguishable by their taste. Another test, and more accurate, is, that acids change the blue juices of vegetables red, and alkalis turn the syrup of violets green.

Acids are generally distinguished according to the three kingdoms to which they belong, viz. _mineral_, _vegetable_, and _animal_. The mineral acids are three, the _nitrous_, the _vitriolic_, and the _marine_.

The nitrous acid is formed by the union of the purest inflammable air, or the purest nitrous air, with dephlogisticated air. But it is usually procured from nitre by means of the vitriolic acid, which, seizing its base, expels the nitrous acid in a liquid form. On this account this acid is said to be weaker than the vitriolic.

If the nitrous acid be made to pa.s.s through a red-hot earthen tube, it will be decomposed, and the greatest part of it be converted into dephlogisticated air.

Like all other acids, the nitrous acid has a strong affinity to water; but it is not capable of so much concentration as the vitriolic. It is generally of an orange or yellow colour; but heat will expel this colour in the form of a red vapour, which is the same acid in the form of air, and loaded with phlogiston; and therefore when it is colourless it is said to be dephlogisticated. But the colourless vapour exposed to heat, or to light, will become coloured again; and the liquid acid imbibing this coloured vapour, becomes coloured as before. This acid tinges the skin of a yellow colour, which does not disappear till the epidermis be changed.

The nitrous acid unites with phlogiston, alkalis, metallic substances, and calcareous earth.

By means of its affinity with phlogiston it occasions that rapid accension called _detonation_, when any salt containing this acid, especially nitre, is applied to hot charcoal, or when charcoal is put to hot nitre. In fact, the charcoal burns so rapidly by means of the dephlogisticated air supplied by the nitre.

A mixture of sulphur a.s.sists the accension of these substances, and makes gunpowder, in the explosion of which much nitrous or phlogisticated air is suddenly produced, and expanded by the heat. The application of this force, both to useful and destructive purposes, is well known. If, instead of nitre, a salt made with dephlogisticated marine acid be made use of, the explosion is more easily produced, and is much more violent. Friction will do this as well as heat.

Nitre also enters into the composition of _pulvis fulminans_, viz. three parts nitre, two of dry alkali, and one of sulphur. This composition melts, and yields a blue flame, before it explodes.

By means of the affinity of the nitrous acid to _oil_, another substance containing phlogiston, it is capable of producing not only a great heat, but even a sudden flame, especially when mixed with a little vitriolic acid.

Nitrous acid dissolves all metallic substances except gold and platina, and in the solution nitrous air is produced.

The particular kinds of saline substances formed by the union of the nitrous acid with the several metals and earths may be seen in tables constructed for the purpose. They are all deliquescent.

LECTURE XIII.

_Of the Vitriolic Acid._

The vitriolic acid, so called because it was originally procured from _vitriol_, is now generally procured from sulphur; the dephlogisticated part of the air uniting with it in the act of burning.

That dephlogisticated air is essential to this acid is evident from the decomposition of it; for if the vapour of it be made to pa.s.s through a red-hot earthen tube, a great quant.i.ty of dephlogisticated air is procured.

This acid has a strong affinity to water, with which it unites with much heat; and it is capable of greater concentration, or of being made specifically heavier, than any other acid. When pure, it is entirely free from colour and smell, owing, probably, to its being free from phlogiston, which is inseparable from the nitrous or marine acids.

The vitriolic acid will dislodge the nitrous, or marine, or any other acid, from their earthy or metallic bases; from which property it is called the strongest of all the acids.

By means of the superior affinity of the vitriolic acid to earths, and especially to _terra ponderosa_, the smallest quant.i.ty of it in water may be discovered by a solution of this earth in the marine acid. In this acid the terra ponderosa is held in perfect solution; but with the vitriolic acid it forms a substance that is insoluble in water, and therefore it instantly appears in the form of a white cloud.

Perhaps chiefly from the strong affinity which this acid has with water, _pyrophorus_, consisting of a mixture of alum and several substances containing phlogiston, takes fire spontaneously on exposure to the air.

It is commonly made of three parts of alum and one of brown sugar, or of two parts alum, one of salt of tartar, and one of charcoal. They must be heated till they have for some time emitted a vapour that burns with a blue flame.

The saline substances produced by the union of this acid with the several earths and metals, are best exhibited in tables constructed for the purpose. When united to three of the metals, viz. iron, copper, and zinc, they are called _vitriols_, green, blue, and white. And all the substances which this acid unites with crystallize, and do not deliquesce.

This acid unites with oil, and the mixture is always black.

When any substance containing phlogiston is heated in the vitriolic acid, another species of the acid, called _sulphureous_, is formed, of a pungent smell. In reality, it is water impregnated with vitriolic acid air. It makes, however, a distinct species of acid, and is dislodged from its base by most of the others.

_Of the Marine Acid._

The marine acid is procured from common salt by the vitriolic acid, which unites with its base, the fossil alkali.

This acid is generally of a straw-colour; but this is owing to an impregnation with some earthy matter, most of which it readily dissolves, especially the metallic ones. It is less capable of concentration than the vitriolic or nitrous acids, perhaps from a more intimate union of phlogiston with it. No heat can extract from it any dephlogisticated air.

Though this is denominated a weaker acid than the nitrous, yet it will take silver, lead, or mercury, from their union with the nitrous acid.

Upon this principle, a solution of these metals in the nitrous acid will readily discover whether any water contains the marine acid, the latter uniting with the metal dissolved in the former, and forming with it, if it be silver, a _luna cornea_; which being a substance insoluble in water, discovers itself by a cloudy appearance.

The union of the marine acid with earths forms salts that easily deliquesce, but with the metals such as are capable of crystallization; and so also is that formed by the union of this acid to terra ponderosa.

Neither this acid nor the nitrous will dissolve gold or platina; but a mixture of them, called _aqua regia_, will do it.