Priestley’s experiments led to no decisive result: he of course recognised the
“peculiar smell, exactly resembling that which is procured by dissolving red lead in the same acids.... On the application of heat it was easy to perceive that air, or vapour, was expelled; but it was instantly seized by the quicksilver.... This is a new field that is yet before me.”
Priestley never occupied that field. It is tolerably certain that both Woulfe and he had unknowingly prepared chlorine gas, but the glory of its discovery belongs to Scheele.
The paper “Of Sound in Different Kinds of Air” is worth quoting as showing Priestley at his best:—
“Almost all the experiments that have hitherto been made relating to sound have been made in common air, of which it is known to be a vibration, though it is likewise known to be capable of being transmitted by other substances. There could be little doubt, however, of the possibility of sound originating in any other kind of air, as well as being transmitted by them; but the trial had not been actually made, and I had an easy opportunity of making it.
“Besides, the experiments promised to ascertain whether the intensity of sound was affected by any other property of the air in which it was made than the mere density of it. For the different kinds of air in which I was able to make the same sound, besides differing in specific gravity, have likewise other remarkable chemical differences, the influence of which with respect to sound would, at the same time, be submitted to examination.
“Being provided with a piece of clock-work, in which was a bell, and a hammer to strike upon it (which I could cover with a receiver, and which, when it was properly covered up, I could set in motion by the pressure of a brass rod going through a collar of leather), I placed it on some soft paper on a transfer. Then taking a receiver, the top of which was closed with a plate of brass, through which the brass rod and collar of leathers was inserted, I placed the whole on the plate of an air-pump, and exhausted the receiver of all the air that it contained. Then removing this exhausted receiver, containing the piece of clock-work, I filled it with some of those kinds of air that are capable of being confined by water.... Then by forcing down the brass rod through the collar of leathers I made the hammer strike the bell, which it would do more than a dozen times after each pressure. And the instrument was contrived to do the same thing many times successively after being once wound up.
“Everything being thus prepared, I had nothing to do, after filling the same receiver with each of the kinds of air in its turn, but receding from the apparatus, while an assistant produced the sound, to observe at what distance I could distinctly hear it. The result of all my observations, as far as I could judge, was that the intensity of sound depends solely upon the density of the air in which it is made, and not at all upon any chemical principle in its constitution.
“In inflammable air the sound of the bell was hardly to be distinguished from the same in a pretty good vacuum; and this air is ten times rarer than common air.
“In fixed air the sound was much louder than in common air, so as to be heard about half as far again; and this air is in about the same proportion denser than common air.
“In dephlogisticated air the sound was also sensibly louder than in common air, and, as I thought, rather more than in the proportion of its superior density; but of this I cannot pretend to be quite sure.
“In all these experiments the common standard was the sound of the same bell in the same receiver, every other circumstance also being the same; the air only being changed by removing the receiver from the transfer and blowing through it, etc.”
The sixth and last volume appeared in 1786 with a dedication to William Constable, Esq., of Barton Constable.
In the preface Priestley is concerned to defend himself against the charge that he occupies himself too much with Theology to the detriment of Natural Philosophy. Theology, he pleads, is his original and proper province, and for which, therefore, he may be allowed to have a justifiable predilection. But as with Metaphysics, so with Theology. Neither subject engrossed so much of his time as some persons imagined.
“I am particularly complained of at present as having thrown away so much time on the composition of my History of the Corruptions of Christianity, and of the Opinions Concerning Christ. But I can assure them, and the nature of the thing, if they consider it, may satisfy them, that the time I must necessarily have bestowed upon the experiments, of which an account is contained in this single volume, is much more than I have given to the six, of which the above-mentioned works consist, and to all the controversial pieces that I have written in defence of the former of them. The labour and attention necessary to enable me to write single paragraphs in this work have been more than was requisite to compose whole sections or chapters of the former.... Besides, these different studies so relieve one another that I believe I do more in each of them, by applying to them alternately, than I should do if I gave my whole attention to one of them only.”
But Priestley’s main defence rests “on the superior dignity and importance of theological studies to any other whatever.” The whole preface must be read in the light of Priestley’s altered circumstances and of his relations to the theological world, which, since his removal to Birmingham, had greatly increased in weight and importance. As already stated, he regarded himself as ordained to champion the cause of religion among the persons to whom his writings as a natural philosopher specially appealed. The author of the Institutes of Natural and Revealed Religion was the writer of the Letters to a Philosophical Unbeliever and, in an age of unbelief, the doughty antagonist of Gibbon. Otherwise the incongruous mixture of Theology and Natural Philosophy, of which the preface is made up, seems inexplicable.