414 MR. T. GRAHAM ON THE ABSORPTION AND 



When the same dialyzing-bag was kept at a temperature of 60° C, the volume of air 

 collected in seven minutes was 6'22 and 7'06 cub. centims. For the square metre this 

 amounts to 6'21 and 7*05, mean 6*65 cub. centims. per minute. The passage of air 

 through rubber is therefore almost exactly three times as quick at 60° as at 20° C. 



Again, the dialyzing-bag was kept at 4° C. by being surrounded by ice and salt. The 

 air now collected in seventy-two minutes was 5*78 and 5'77 cub. centims. in volume — 

 for a square metre 0'56 cub. centim. per minute. The passage of air through rubber 

 thus appears to be four times as slow at 4° C. as it is at 20°. The proportion of oxygen 

 in the dialyzed air increased at the same time. In the two portions of air collected at 

 4° the oxygen was 46-75 and 47'43 per cent. The increase of oxygen at a low tempe- 

 rature was confirmed in other experiments ; but it appeared at the same time that the 

 rubber was liable to acquire a true porosity to a slight extent when retained for some 

 hours about 0° C. The rubber then allowed air to pass through it containing no more 

 than 28 or even 23 per cent, of oxygen, and in volume still very small. The rubber 

 has become rigid by the cold, and is now acting feebly as a porous substance, allowing 

 a little gas-difi\ision to take place through its substance. Such a condition, which is 

 accidental to caoutchouc at a low temperature, appears to be constant with gutta percha, 

 a harder material, at 20° C, and even higher temperatures. 



A large bag of varnished silk with a surface of 1-672 square metre (two square yards) 

 was found still more convenient. It was, however, rather beyond the exhausting-power 

 of the largest Sprengel pump. It yielded in eight minutes, without any collecting- 

 flask, 22, 21-55, and 21-5, mean 21-68 cub. centims. This was a supply of 2-71 cub. 

 centims. per minute, and was at the rate, for a square metre, of 1-62 cub. centim. per 

 minute. The supply would have been about a half more if the dialyzed air had not 

 gained upon the pump. The air of the first and last observations contained respectively 

 41-89 and 41*85 per cent, of oxygen. 



The usual proportion of oxygen in air dialyzed by rubber appears to be about 41 '6 

 per cent.; and it may be described as atmospheric air deprived of one-half of its usual 

 proportion of nitrogen. A single dialysis of air therefore carries the experimenter 

 already halfway from air to pure oxygen as the final result. But the gain by a second 

 dialysis could not be so great, as it would only withdraw one-half of the nitrogen that 

 remained after the first operation, a third dialysis one-half of the nitrogen remaining 

 after the second operation, and so on — each step of the concentration of the oxygen 

 being obtained at a greater cost than the last, and the best conceivable result being only 

 a good approximation. The practical problem which is suggested by the air-dialyzer 

 is, to attain the means on a large scale of reducing to one-half, or so, the proportion of 

 nitrogen in atmospheric air, to be applied to certain useful purposes. 



6. Percolation of air through gutta percha and other septa. 



Thin transparent sheets of a certain material represented as air- and water-tight are 

 in common use. It is often spoken of as consisting of caoutchouc, but appears to have 



