“These appearances I explain by supposing that the vitriolic acid, in uniting with the spar, is in part volatilised by means of some phlogiston contained in it, so as to form a vitriolic acid air; and there is also combined with this air a portion of the solid earthy part of the spar, which continues in a state of solution till, coming into contact with the water, the fluid unites with the acid, and the earth is precipitated.”
The third volume of the work was published in the early part of 1777, with a dedication to Lord Stanhope. It opens, as usual, with the characteristically discursive preface, extending to thirty pages, in which the author apologises for the character of much in the volume. He is constrained to admit that numerous as his facts are, “few of them will appear so brilliant in the eye of the general scholar” as in either of the two former volumes, although he trusts they will “be thought no less valuable by philosophers and chemists.” Priestley, it would seem, was conscious that he was beginning, as the phrase goes, “to write himself out.”
“Lest my readers should be alarmed at this addition of one volume after another on the same subject, I do assure them that I shall now certainly give them and myself some respite, and deliver the torch to anyone who may be disposed to carry it, foreseeing that my attention will be sufficiently engaged by speculations of a very different nature.... It will be a great satisfaction to me, after the part that I have taken in this business, to be a spectator of its future progress, when I see the work in so many and so good hands, and everything in so rapid and so promising a way.
“On taking leave of this subject I would entreat the candour and indulgence of my readers for any oversights they may discover in me as a philosopher, or imperfections as a writer. I am far from pretending to infallibility; but I have the satisfaction to reflect that, imperfect as my works may be found to be, they are each as perfect as I was able to make them....
“Upon this, as upon other occasions, I can only repeat that it is not my opinions on which I would be understood to lay any stress. Let the new facts, from which I deduce them, be considered as my discoveries, and let other persons draw better inferences from them if they can. This is a new and a wide field of experiment and speculation, and a premature attachment to hypothesis is the greatest obstruction we are likely to meet with in our progress through it; and as I think I have been pretty much upon my guard myself, I would caution others to be upon their guard too.”
These passages evidently were written under the influence of the feeling of resentment with which he viewed the criticism to which his speculations were subjected abroad. Fontana, Lavoisier and others were, indeed, zealously engaged in using Priestley’s own facts to destroy the conception by which he explained them. An appeal to the balance was felt to be necessary, and Priestley, as a logician, could not resist it. But he was no quantitative chemist: the habits of a Cavendish were quite foreign to his genius: patient, scrupulous attention to numerical accuracy was not one of his characteristics: he was one of the most industrious of experimenters—delighting, indeed, in manipulation for the mere sake of it, but withal hasty and superficial. It is nowhere evident in his writings that his problems were attacked according to any carefully-thought-out plan. He confesses indeed, on more than one occasion, he tested the inflammability of one of his numerous “airs” because he had a lighted candle near him: had the candle not been lighted it would not have occurred to him to do it. Priestley was, in fact, a pioneer: he showed the existence of a new world for science, and he himself roamed over a portion of it, like a second Joshua; but he had not the experience or the aptitude to accurately map out even that fraction.
There is little in the third volume of permanent value. It is largely an account of a series of disconnected observations on the action of nitric acid upon a variety of substances, which, however, led to no general conclusions. It is, however, certain that if Priestley could have induced himself to follow up certain of his observations he would have arrived at facts of far greater importance than those he actually narrates. “Speculation,” he said, by way of rejoinder to Lavoisier, “is a cheap commodity. New and important facts are most wanted, and therefore of most value,” and the new and important facts were within his grasp if he had only reached out for them.
Another portion of the work is concerned with supplementary observations on the gases treated of in the preceding volumes, partly by way of correction and partly additional. Here and there we have a suggestive passage, as in the paper on “Experiments on the Mixture of Different Kinds of Air that have no Mutual Action,” in which he thus clearly indicates the principle of the intra-diffusion of gases.
“The result of my trials has been this general conclusion: that when two kinds of air have been mixed it is not possible to separate them again by any method of decanting or pouring them off, though the greatest possible care be taken in doing it. They may not properly incorporate, so as to form a third species of air, possessed of new properties; but they will remain equally diffused through the mass of each other; and whether it be the upper or the lower part of the air that is taken out of the vessel, without disturbing the rest, it will contain an equal mixture of them both.”
Another suggestive paper is on “Respiration and the Use of the Blood,” which was read to the Royal Society on January 25, 1776, and appears in the Phil. Trans., vol. lxvi. Priestley, of course, regarded respiration as a phlogistic process, and “that the use of the lungs is to carry off a putrid effluvium, or to discharge that phlogiston, which had been taken into the system with the aliment, and has become, as it were, effete, the air that is respired serving as a menstruum for that purpose.” This he thinks he has “proved to be effected by means of the blood, in consequence of its coming so nearly into contact with the air in the lungs, the blood appearing to be a fluid wonderfully formed to imbibe and part with that principle which the chemists call phlogiston, and changing its colour in consequence of being charged with it or being freed from it.” The facts in this paper are for the most part correctly stated, but the discoverer of oxygen led the world woefully astray as to the part played by that gas in the phenomena of respiration.
The fourth volume made its appearance in March 1779, with a dedication to Sir George Savile, who had rendered Priestley the service of introducing him and his invention of soda-water to the notice of the Admiralty. In the preface, which is commendably short, he makes some reference to the respite which he had promised himself and his readers, but trusts, by way of extenuation, “it may be sufficient to allege the instability of human purposes and pursuits.” He had intended to devote himself to metaphysics.
“But that kind of writing,” he says, “is a thing of a very different nature from this. I can truly say ... that single sections in this work have cost me more than whole volumes of the other; so great is the difference between writing from the head only and writing, as it may be called, from the hands.”