ACIDS are a class of chemical agents, distinguished by the Follow ing common properties :—They have a sour taste, and are the only substances to which this taste belongs ; they arc more or less soluble in water; they change the blue, purple, and green colours of vegetables to a red ; combine N\ id] alkalis and earths, neutralizing their common properties ; and dissolve metals and me tallic oxides. These properties are possessed by the different acids in very different degrees. In some of them, the sour taste is scarcely perceptible in their most concentrated state ; in others, it is extremely strong, even when they are largely diluted with water : and a similar diversity exists with regard to all their other characteristic properties. In general, they exert ener getic chemical actions, partly from the facility with which they afford oxygen to bodies, and partly from the pow erful affinities they directly exert; and the history of their properties and combinations constitutes the most extensive department of the science of chemistry.
The important truth, that oxygen is the principle of acidity, was established by the researches of Lavoisier, and formed one of the leading propositions of the mo dern chemical system, which he contributed to rear. By a very ample induction, this eminent chemist demon strated, that the greater number of simple inflammable substances, when combined with oxygen, in certain pro portions, form compounds possessed of acid powers ; and again, by decomposing a number of acids, he proved, that oxygen exists in them as a constituent principle. Ile hence inferred, that oxygen is essential to their com position, and is the principle of There existed, even at the period of Lavoisier•s re searches, some objection to this conclusion. Three powerful acids had, in particular, not been decomposed ; and it w as therefore only an hypothesis to infer, as it was necessary to do, in conformity to the theory he sought to establish, that they are compounds, and contain oxygen as an clement. Yet the analogy between these, and the other acids which had been decomposed and proved to contain oxygen, is so direct, that the inference could scarcely but be admitted as having much probability ; and it is probable that it will soon be confirmed by the application of Gals anism to the analysis of these acids.
There are some other facts which have been stated as unfavourable to the conclusion, that oxygen is exclusive ly the principle of acidity. Thus it is affirmed, that there arc acids, the composition of which is known, and in which no oxygen is contained. Sulphuretted hydrogen
gas has been given as an example of this. It is a com pound of sulphur and hydrogen, in the proportions, as determined by Thenard, of 70.857 of sulphur, and 29.14.3 of hydrogen. Yet it appears to possess the properties of an acid. It changes the more delicate vegetable co lours, as those of litmus and radish, to a red, it is abun dantly soluble in water, decomposes soap, a property be longing to the acids, and, what is the most distinctive acid character, combines with the alkalis, earths, and metallic oxides, forming compounds, some of which are crystallizable, and not very dissimilar in their properties to the compounds of these substances with the acids. llence Berthollet has not hesitated to rank it as an acid, and to conclude that it proves, " that acidity is not in all cases owing to oxygen."' The is still, hoYvevcr, some reason to doubt w ith re • Bard to this conclusion. Though this sohstance reddelni of the vegetable colours, there are others on ee it has not this effect; and one of them, the syrup of vio lets, reddened by all the acids, it even changes to a green. And Berthollet himseil has admitted, that he is uncertain if its power is sufficiently energetic to pro duce neutralization in its combinations with the alkaline bases. Besides, if even its acid powers were unequivo cally established, its composition is not determined with that precision which would pre( lode the possibility of oxygen being contained in it. In all the processes by which it is formed, oxygen is present, ;mil may enter into combination with its other elements; and in all the analyses of it hitherto executed, it will he found, that no attention has been paid to the possible source of fallacy, that, if it contain oxygen, this, when its composition is subverted, may combine with a portion of its hydrogen, and form water. The proportions of its elements have always been estimated from experiments, in which the proportion of sulphur only was directly determined. From this was inferred, the proportion of hydrogen, w ith which this sulphur was combined. But it is obvious, that for any thing proved by such an analysis, oxygen may have also been a constituent principle of it, amid the proportion of hydrogen estimated too high. And if oxygen do enter into its composition, a small quantity may communicate to it its weak acid powers ; for we are not to judge of the quantity of oxygen necessary to acidify a compound base, from the quantity that would be required to render acid either of its elements.