To change this air I filled the tube with quicksilver, and pouring it out again, I replaced the tube in its former situation; when the air was diminished again, but not so fast as before. The same lining of cement diminished the air a third time. How long it will retain this power I cannot tell. This cement had been made several months before I made this experiment with it. I must observe, however, that another quantity of this kind of cement, made with a finer and more liquid turpentine, had not the power of diminishing air, except in a very small proportion. Also the common red cement has this property in the same small degree. Common air, however, which had been confined in a glass vessel lined with this cement about a month, was so far injured that a candle would not burn in it. In a longer time it would, I doubt not, have become thoroughly noxious. Iron that has been suffered to rust in nitrous air diminishes common air very fast, as I shall have occasion to mention when I give a continuation of my experiments on nitrous air. Lastly, the same effect, I find, is produced by the _electric spark_, though I had no expectation of this event when I made the experiment. This experiment, however, and those which I have made in pursuance of it, has fully confirmed another of my conjectures, which relates to the _manner_ in which air is diminished by being overcharged with phlogiston, viz. the phlogiston having a nearer affinity with some of the constituent parts of the air than the fixed air which enters into the composition of it, in consequence of which the fixed air is precipitated. This I first imagined from perceiving that lime-water became turbid by burning candles over it, p. 44. This was also the case with lime-water confined in air in which an animal substance was putrefying, or in which an animal died, p. 79. and that in which charcoal was burned, p. 81. But, in all these cases, there was a possibility of the fixed air being discharged from the candle, the putrefying substance, the lungs of the animal, or the charcoal. That there is a precipitation of lime when nitrous air is mixed with common air, I had not then observed, but I have since found it to be the case. That there was no precipitation of lime when brimstone was burned, I observed, p. 45. might be owing to the fixed air and the lime uniting with the vitriolic acid, and making a salt, which was soluble in water; which salt I, indeed, discovered by the evaporation of the water. I also observed, p. 46, 105. that diminished air being rather lighter than common air is a circumstance in favour of the fixed, or the heavier part of the common air, having been precipitated.