A piece of _alum_ exposed to this air turned yellow, absorbed it as fast as the saltpetre had done, and was reduced by it to the form of a powder. Common salt, as might be expected, had no effect whatever on this marine acid air. I had also imagined, that if air diminished by the processes above-mentioned was affected in this manner, in consequence of its being saturated with phlogiston, a mixture of this acid air might imbibe that phlogiston, and render it wholesome again; but I put about one fourth of this air to a quantity of air in which metals had been calcined, without making any sensible alteration in it. I do not, however, infer from this, that air is not diminished by means of phlogiston, since the common air, like some other substances, may hold the phlogiston too fast, to be deprived of it by this acid air. I shall conclude my account of these experiments with observing, that the electric spark is visible in acid air, exactly as it is in common air; and though I kept making this spark a considerable time in a quantity of it, I did not perceive that any sensible alteration was made in it. A little inflammable air was produced, but not more than might have come from the two iron nails which I made use of in taking the sparks. FOOTNOTES: [9] It will be seen, in the second part of this work, that, in some of these processes, I had afterwards more success. SECTION X. MISCELLANEOUS OBSERVATIONS. 1. As many of the preceding observations relate to the _vinous_ and _putrefactive_ fermentations, I had the curiosity to endeavour to ascertain in what manner the air would be affected by the _acetous_ fermentation. For this purpose I inclosed a phial full of small beer in a jar standing in water; and observed that, during the first two or three days, there was an increase of the air in the jar, but from that time it gradually decreased, till at length there appeared to be a diminution of about one tenth of the whole quantity. During this time the whole surface of it was gradually covered with a scum, beautifully corrugated. After this there was an increase of the air till there was more than the original quantity; but this must have been fixed air, not incorporated with the rest of the mass; for, withdrawing the beer, which I found to be sour, after it had stood 18 or 20 days under the jar, and passing the air several times through cold water, the original quantity was diminished about one ninth. In the remainder a candle would not burn, and a mouse would have died presently. The smell of this air was exceedingly pungent, but different from that of the putrid effluvium. A mouse lived perfectly well in this air, thus affected with the acetous fermentation; after it had stood several days mixed with four times the quantity of fixed air.