“Another thing which Mr. Lavoisier endeavours to prove is that dephlogisticated air is the acidifying principle. From what has been explained, it appears that this is no more than saying that acids lose their acidity by uniting to phlogiston, which, with regard to the nitrous, vitriolic, phosphoric, and arsenical acids, is certainly true”. “But as to the marine acid and acid of tartar, it does not appear that they are capable of losing their acidity by any union with phlogiston.” Here Cavendish does not consider the question of gain of weight on loss of phlogiston, or if he does, he must ascribe it to simultaneous entry of water. And experimental research at that time was not far enough advanced to enable him to decide finally as to the truth of this hypothesis. In his next memoir, read before the Royal Society on June 2nd, 1785, Cavendish relates experiments on the passage of electric sparks through air, the experiment having first been tried by Priestley. Priestley says:[24]--“Lastly, the same effect [_i.e._ the diminution of the volume of common air], I find, is produced by the _electric spark_, though I had no expectation of this event when I made the experiment.” And again:--“At the time of my former publication, I had found that taking the _electric spark_ in given quantities of several kinds of air had a very remarkable effect on them, that it diminished common air and made it noxious, making it deposit its fixed air exactly like any phlogistic process; from whence I concluded that the electric matter either is or contains phlogiston.” Cavendish had mentioned this process casually as one of the methods of phlogisticating air; in beginning his second paper he says:--“I now find that, though I was right in supposing the phlogistication of the air does not proceed from phlogiston communicated to it by the electric spark, and that no part of the air is converted into fixed air; yet that the real cause of the diminution is very different from what I suspected, and depends upon the conversion of phlogisticated air into nitrous acid.” The apparatus he used was very simple. It consisted of a glass siphon filled with mercury, each leg dipping into a glass likewise containing mercury; the air was admitted by a gas-pipette into the bend of the siphon, and on connecting the mercury in one of the glasses with a ball placed near the prime conductor of an electric machine, and the other with the earth, sparks could be made to pass from the mercury in one limb to that in the other.