VOL. LXXVU.] PHILOSOPHICAL TRANSACTIONS. 207 



produced in the experiments with raw silk, and in those with poplar cotton, Sir 

 B. made the following experiments, viz. Exper. 17, 18-25, with nearly similar 

 results. On which he then remarks, that the water in several of these experi- 

 ments had acquired a faint greenish cast ; but the colour of that with the cotton 

 was rather the deeper. On examining the water of the cotton under a micros- 

 cope, I found it contained a great number of animalcules, exceedingly small, 

 and of nearly a round figure. That with the silk contained the same kind of 

 animalcules also, but not in so great abundance. I never failed to find them in 

 every case in which the water used in an experiment had acquired a greenish hue ; 

 and from their presence alone, I think it more than probable, that the colour of 

 the water, in the first instance, arose in all cases. I was yet by no means satis- 

 fied with respect to the part which the silk and other bodies, exposed in water 

 in the foregoing experiments, acted in the purifying or dephlogisticating of the 

 air produced. Dr. Priestley has long since discovered, that many animal ?.nd 

 vegetable substances putrifying, or rather dissolving, in water, in the sun, cause 

 the water to yield large quantities of dephlogisticated air ; but I could hardly 

 conceive, that the small quantity of silk which was used in my experiments, and 

 which had been constantly in water for more than 3 months, and had so often 

 been washed, and even boiled in water, should yet retain a power of communi- 

 cating any thing to the large quantities of fresh water in which it was successively 

 placed ; at least any thing in sufficient quantities to impregnate those bodies of 

 water, and to cause them to yield the great abundance of air whicn they 

 produced. 



It was still more difficult to account for the purification of the air in the expe- 

 riments with wool and fur, and human hair ; especially, as in some of these 

 experiments the water had not sensibly changed colour, nor did it appear to have 

 lost any thing of its transparency. It is true, in these cases, the quantities of 

 air produced were very small ; but yet its quality was better than that of common 

 air, and considerably superior to that of the air existing in the water, previous 

 to its being exposed to the action of the sun's light. In short, it was dephlo- 

 gisticated in the experiment ; but the manner in which this was done is very 

 difficult to ascertain. With a view to throw some new light on this intri- 

 cate subject, I made the following experiments. 



Exper. 26. Concluding that if silk and other bodies, used in the foregoing 

 experiments, actually did not contribute any thing, considered as chemical sub- 

 stances, in the process of the production of pure air yielded by water ; but if, 

 on the contrary, they acted merely as a mechanical aid in the separation of the 

 air from the water, by affiDrding a convenient surface for the air to attach itself 

 to; in this case, any other body, having a large surface, and attracting air in 

 water, might be used instead of silk in the experiment, and pure air would be 



