Heroes of Science: Chemists

by M. M. Pattison Muir

Acids, alkalis and salts all contain, according to Stahl (beginning of the eighteenth century), more or less primordial acid. The more of this a substance contains, the more acid it is; the less of this it contains, the more alkaline it is.

All these attempted explanations recognize that similar properties are to be traced to similarity of composition; but the assertion of the existence of a "primordial acid," or of "the matter of fire," although undoubtedly a step in advance, was not sufficiently definite (unless it was supplemented by a distinct account of the properties of these principles) to be accepted when chemical knowledge became accurate.

The same general consideration, founded on a large accumulation of facts, viz. that similarity of properties is due to similarity of composition, guided Lavoisier in his work on acids. He found the "primordial acid" of Stahl, and the "biting principle" of Meyer, in the element oxygen.

I have already (p. 91) shortly traced the reasoning whereby Lavoisier arrived at the conclusion that oxygen is the acid-producer; here I would insist on the difference between his method and that of Basil Valentine, Stahl and the older chemists. They carried into the domain of natural science conceptions obtained from, and essentially belonging to the domain of metaphysical or extra-physical speculation; he said that oxygen is the acidifier, because all the compounds of this element which he actually examined were possessed of the properties included under the name acid. We know that Lavoisier's conclusion was erroneous, that it was not founded on a sufficiently broad basis of facts. The conception of an acidifying principle, although that principle was identified with a known element, was still tainted with the vices of the alchemical school. We shall see immediately how much harm was done by the assertion of Lavoisier, "All acids contain oxygen."

In Chapter II. (pp. 32-37) we traced the progress of knowledge regarding alkalis from the time when the properties of these bodies were said to be due to the existence in them of "matter of fire," to the time when Black had clearly distinguished and defined caustic alkali and carbonated alkali.

The truly philosophical character, and at the same time the want of enthusiasm, of Black become apparent if we contrast his work on alkali with that of Lavoisier on acid. Black did not hamper the advance of chemistry by finding a "principle of alkalinity;" but neither did he give a full explanation of the fact that certain bodies are alkaline while others are not. He set himself the problem of accurately determining the differences in composition between burnt (or caustic) and unburnt (or mild) alkali, and he solved the problem most successfully. He showed that the properties of mild alkalis differ from those of caustic alkalis, because the composition of the former differs from that of the latter; and he showed exactly wherein this difference of composition consists, viz. in the possession or non-possession of fixed air.

Strange we may say that this discovery did not induce Black to prosecute the study of caustic alkalis: surely he would have anticipated Davy, and have been known as the discoverer of potassium and sodium.

In the time of Stahl the name "salt" was applied, as we have learned, to the substance produced by the union of an acid with an alkali; but the same word was used by the alchemists with an altogether different signification. Originally applied to the solid matter obtained by boiling down sea-water, and then extended to include all substances which, like this solid matter, are very easily dissolved by water and can be recovered by boiling down this solution, "salt" was, in the sixteenth and seventeenth centuries, the name given to one of the hypothetical principles or elements. Many kinds of matter were known to be easily dissolved by water; the common possession of these properties was sought to be accounted for by saying that all these substances contained the same principle, namely, the principle of salt. I have already tried to indicate the reasoning whereby Boyle did so much to overthrow this conception of salt. He also extended our knowledge of special substances which are now classed as salts. The chemists who came after Boyle gradually reverted to the older meaning of the term "salt," adopting as the characteristics of all substances placed in this class, ready solubility in water, fusibility, or sometimes volatility, and the possession of a taste more or less like that of sea-salt.

Substances which resembled salts in general appearance, but were insoluble in water, and very fixed in the fire, were called "earths"; and, as was generally done in those days, the existence of a primordial earth was assumed, more or less of which was supposed to be present in actual earths. This recognition of the possibility of more or less of the primordial earth being present in actually occurring earths, of course necessitated the existence of various kinds of earth. The earths were gradually distinguished from each other; lime was recognized as a substance distinct from baryta, baryta as distinct from alumina, etc.

Stahl taught that one essential property of an earth was fusibility by fire, with production of a substance more or less like glass. This property was possessed in a remarkable degree by quartz or silica. Hence silica was regarded as the typical earth, until Berzelius, in 1815, proved it to be an acid. But the earths resembled alkalis, inasmuch as they too combined with, and so neutralized, acids.