“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.