Part 12 (1/2)
”In Nature,” says Wordsworth, ”everything is distinct, but nothing defined into absolute independent singleness.”
At this stage of advance, then, an earth is regarded as differing from an alkali in being insoluble, or nearly insoluble in water; in not being soapy to the touch, and not turning vegetable reds to blue: but as resembling an alkali, in that it combines with and neutralizes an acid; and the product of this neutralization, whether accomplished by an alkali or by an earth, is called a salt. To the earth or alkali, as being the foundation on which the salt is built, by the addition of acid, the name of _base_ was given by Rouelle in 1744.
But running through every conception which was formed of these substances--acid, alkali, earth, salt--we find a tendency, sometimes forcibly marked, sometimes feebly indicated, but always present, to consider salt as a term of much wider acceptation than any of the others.
An acid and an alkali, or an acid and an earth, combine to form a salt; but the salt could not have been thus produced unless the acid, the alkali and the earth had contained in themselves some properties which, when combined, form the properties of the salt.
The acid, the alkali, the earth, each is, in a sense, a salt. The perfect salt is produced by the coalescence of the saltness of the acid with the saltness of the alkali. This conception finds full utterance in the names, once in common use, of _sal acidum_ for acid, _sal alkali_ for alkali, and _sal salsum_ or _sal neutrum_ for salt. All are salts; at one extreme comes that salt which is marked by properties called acid properties, at the other extreme comes the salt distinguished by alkaline properties, and between these, and formed by the union of these, comes the middle or neutral salt.
It is thus that the nomenclature of chemistry marks the advances made in the science. ”What's in a name?” To the historical student of science, almost everything.
We shall find how different is the meaning attached in modern chemistry to these terms, _acid salt_, _alkaline salt_, _neutral salt_, from that which our predecessors gave to their _sal acidum_, _sal alkali_, and _sal neutrum_.
We must note the appearance of the term _vitriol_, applied to the solid salt-like bodies obtained from acids and characterized by a gla.s.sy l.u.s.tre.
By the middle of last century the vitriols were recognized as all derived from, or compounded of, sulphuric acid (oil of vitriol) and metals; this led to a subdivision of the large cla.s.s of neutral salts into (1) metallic salts produced by the action of sulphuric acid on metals, and (2) neutral salts produced by the action of earths or alkalis on acids generally.
To Rouelle, a predecessor of Lavoisier, who died four years before the discovery of oxygen, we owe many accurate and suggestive remarks and experiments bearing on the term ”salt.” I have already mentioned that it was he who applied the word ”base” to the alkali or earth, or it might be metal, from which, by the action of acid, a salt is built up. He also ceased to speak of an acid as _sal acidum_, or of an alkali as _sal alkali_, and applied the term ”salt” exclusively to those substances which are produced by the action of acids on bases. When the product of such an action was neutral--that is, had no sour taste, no soapy feeling to the touch, no action on vegetable colours, and no action on acids or bases--he called that product _a neutral salt_; when the product still exhibited some of the properties of acid, _e.g._ sourness of taste, he called it _an acid salt_; and when the product continued to exhibit some of the properties of alkali, _e.g._ turned vegetable reds to blue, he called it _an alkaline salt_.
Rouelle also proved experimentally that an acid salt contains more acid--relatively to the same amount of base--than a neutral salt, and that an alkaline salt contains more base--relatively to the same amount of acid--than a neutral salt; and he proved that this excess of acid, or of base, is chemically united to the rest of the salt--is, in other words, an essential part of the salt, from which it cannot be removed without changing the properties of the whole.
But we have not as yet got to know why certain qualities connoted by the term ”acid” can be affirmed to belong to a group of bodies, why certain other, ”alkaline,” properties belong to another group, nor why a third group can be distinguished from both of these by the possession of properties which we sum up in the term ”earthy.” Surely there must be some peculiarity in the composition of these substances, common to all, by virtue of which all are acid. The atom of an acid is surely composed of certain elements which are never found in the atom of an alkali or an earth; or perhaps the difference lies in the number, rather than in the nature of the elements in the acid atoms, or even in the arrangement of the elementary atoms in the compound atom of acid, of alkali, and of earth.
I think that our knowledge of salt is now more complete than our knowledge of either acid, alkali, or earth. We know that a salt is formed by the union of an acid and an alkali or earth; if, then, we get to know the composition of acids and bases (_i.e._ alkalis and earths), we shall be well on the way towards knowing the composition of salts.
And now we must resume our story where we left it at p. 176. Lavoisier had recognized oxygen as the acidifier; Black had proved that a caustic alkali does not contain carbonic acid.
Up to this time metallic calces, and for the most part alkalis and earths also, had been regarded as elementary substances. Lavoisier however proved calces to be compounds of metals and oxygen; but as some of those calces had all the properties which characterized earths, it seemed probable that all earths are metallic oxides, and if all earths, most likely all alkalis also. Many attempts were made to decompose earths and alkalis, and to obtain the metal, the oxide of which the earth or the alkali was supposed to be. One chemist thought he had obtained a metal by heating the earth baryta with charcoal, but from the properties of his metal we know that he had not worked with a pure specimen of baryta, and that his supposed metallic base of baryta was simply a little iron or other metal, previously present in the baryta, or charcoal, or crucible which he employed.
But if Lavoisier's view were correct--if all bases contained oxygen--it followed that all salts are oxygen compounds. Acids all contain oxygen, said Lavoisier; this was soon regarded as one of the fundamental facts of chemistry. Earths and alkalis are probably oxides of metals; this before long became an article of faith with all orthodox chemists. Salts are produced by the union of acids and bases, therefore all salts contain oxygen: the conclusion was readily adopted by almost every one.
When the controversy between Lavoisier and the phlogistic chemists was at its height, the followers of Stahl had taunted Lavoisier with being unable to explain the production of hydrogen (or phlogiston as they thought) during the solution of metals in acids; but when Lavoisier learned the composition of water, he had an answer sufficient to quell these taunts.
The metal, said Lavoisier, decomposes the water which is always present along with the acid, hydrogen is thus evolved, and the metallic calx or oxide so produced dissolves in the acid and forms a salt. If this explanation were correct--and there was an immense ma.s.s of evidence in its favour and apparently none against it--then all the salts produced by the action of acids on metals necessarily contained oxygen.
The Lavoisierian view of a salt, as a compound of a metallic oxide--or base--with a non-metallic oxide--or acid--seemed the only explanation which could be accepted by any reasonable chemist: in the early years of this century it reigned supreme.
But even during the lifetime of its founder this theory was opposed and opposed by the logic of facts. In 1787 Berthollet published an account of experiments on prussic acid,--the existence and preparation (from Prussian blue) of which acid had been demonstrated three or four years before by the Swedish chemist Scheele--which led him to conclude this compound to be a true acid, but free from oxygen. In 1796 the same chemist studied the composition and properties of sulphuretted hydrogen, and p.r.o.nounced this body to be an acid containing no oxygen.
But the experiments and reasoning of Berthollet were hidden by the ma.s.ses of facts and the cogency of argument of the Lavoisierian chemists.
The prevalent views regarding acids and bases were greatly strengthened by the earlier researches of Sir Humphry Davy, in which he employed the voltaic battery as an instrument in chemical investigation. Let us now consider some of the electro-chemical work of this brilliant chemist.
In the spring of the year 1800 the electrical battery, which had recently been discovered by Volta, was applied by Nicholson and Carlisle to effect the decomposition of water. The experiments of these naturalists were repeated and confirmed by Davy, then resident at Bristol, who followed up this application of electricity to effect chemical changes by a series of experiments extending from 1800 to 1806, and culminating in the Bakerian Lecture delivered before the Royal Society in the latter year.
The history of Davy's life during these years, years rich in results of the utmost importance to chemical science, will be traced in the sequel; meanwhile we are concerned only with the results of his chemical work.
The first Bakerian Lecture of Humphry Davy, ”On some Chemical Agencies of Electricity,” deserves the careful study of all who are interested in the methods of natural science; it is a brilliant example of the disentanglement of a complex natural problem.
Volta and others had subjected water to the action of a current of electricity, and had noticed the appearance of acid and alkali at the oppositely electrified metallic surfaces. According to some experimenters, the acid was nitrous, according to others, muriatic acid. One chemist a.s.serted the production of a new and peculiar body which he called _the electric acid_. The alkali was generally said to be ammonia.
When Davy pa.s.sed an electric current through distilled water contained in gla.s.s vessels, connected by pieces of moist bladder, cotton fibre, or other vegetable matters, he found that nitric and hydrochloric acids were formed in the water surrounding the positively electrified plate or pole, and soda around the negatively electrified pole, of the battery.