The remaining papers call for little comment. In the course of some further inquiries Priestley discovered sulphuretted hydrogen, termed by him sulphurated inflammable air, and which he prepared by the action of oil of vitriol upon ferrous sulphide. This gas must of course have been frequently obtained or perceived by him, and possibly by others, as it is produced by a number of processes. Its characteristic smell was associated with sulphur: it was thought to be nothing but inflammable air modified or polluted by the accidental presence of sulphur. It cannot be held that Priestley drew the same sharp distinctions between the various kinds of inflammable air that we draw to-day. To us they are essentially different substances. Priestley, however, regarded them as in the main phlogiston combined or associated with other substances which affected the character of their flames or gave them different properties. In his opinion they were essentially the same. This fact serves to explain what is otherwise incomprehensible, and accounts for many of his mistakes.
The last paper in the volume, excluding the “Supplementary Observations,” has a special interest. It is entitled “Observations relating to Theory,” and is in fact Priestley’s Confession of Faith in the doctrine which enslaved and misled him throughout the whole of his scientific career. But he makes it so hesitatingly and with so many reservations that one wonders why he is constrained to make it at all. He appears to think, however, that it is expected of him.
“It is always our endeavour, after making experiments, to generalise the conclusions we draw from them, and by this means to form a theory, or system of principles, to which all the facts may be reduced, and by means of which we may be able to foretell the results of future experiments.... In my former publications I have frequently promised to give such a general theory of the experiments in which the different kinds of air are concerned, as the present state of our knowledge of them will enable me to do. But, like Simonides with respect to the question that was proposed to him concerning God, I have deferred it from time to time; and indeed I am more than ever disposed to defer it still longer, as I own that I am at present even less able to give such a theory as shall satisfy myself than I was some years ago; new difficulties having arisen, which unhinge former theories, and more experiments being necessary to establish new ones.
“Fluctuating, however, as the present state of this branch of knowledge is, I do not think that I can, on this occasion, entirely decline giving some observations of a theoretical nature, and though I cannot pretend to perform the whole of my promise, I shall give a summary view of what appears to me to be the constituent parts of all the kinds of air with which we are acquainted, and a more particular account of the hypothesis concerning phlogiston, which is at present more an object of discussion than anything else of a theoretical nature.”
Priestley then passes in review all the “airs” of which the chemistry of his time had any knowledge, giving the elements or constituent principles of which he imagined them to be composed.
The only kind of air that he thinks to be properly elementary, and to consist of a simple substance, is dephlogisticated air, with possibly the addition of the principle of heat, which, as it is not probable that it adds to the weight of bodies, can hardly be called an element in their composition.
“Dephlogisticated air appears to be one of the elements of water, of fixed air, of all the acids, and of many other substances which, till lately, have been thought to be simple. The air of the atmosphere, exclusive of a great variety of foreign impregnations, appears to consist of dephlogisticated and phlogisticated air.”
As regards phlogisticated air—the mephitic air of Rutherford, the azote of Lavoisier, the nitrogen of Chaptal—Priestley, reasoning from Cavendish’s work, concluded that it was probably not elementary, but “that it consists of nitrous acid and phlogiston; this acid having always been produced by decomposing it with ... dephlogisticated air.”
He is conscious, however, of the insufficiency of this hypothesis, and suggests
“that the acid principle is supplied by the dephlogisticated air, while the nitrous air gives the base of the nitrous acid and phlogiston; and then this [phlogisticated] air may perhaps be considered as phlogiston combined not with all the necessary elements of nitrous acid, but only what may be called the base of it, viz., the dephlogisticated nitrous vapour, or something which when united to dephlogisticated air will constitute nitrous acid.”
“Fixed air (carbonic acid) seems to be a compound of phlogiston and dephlogisticated air.” In other words, carbonic acid and water have, according to Priestley, “the same elementary composition.” “It is something remarkable that two substances so different from each other as fixed air and water should be analysed into the same principles. But there is this difference between them, that water is the union not of pure phlogiston but of inflammable air and dephlogisticated air.”