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.