OXYGEN AND ITS SALINE COMBINATIONS 



155 



oxygen may be separated from air by causing it to combine with sub- 

 stances which may be easily decomposed by the action of heat, and, in 



rubber. This may be done in the following way : A common india-rubber cushion, E 



(Fig. 27), is taken, and its orifice hermetically connected with an air-pump, or, better 



still, a mercury aspirator (the Sprengel pump is designated by the letters A, c, B). "When 



the aspirator (Chap. II. note 16) 



pumps out the air, which will be 



seen by the mercury running 



out in an almost uninterrupted 



stream, and from its stand- 



ing at near the barometric 



height, then it may be clearly re- 



marked that gas passes through 



the india-rubber. This is also 



seen from the fact that bubbles 



of gas continually pass along with 



the mercury. A small pressure 



of air may be constantly kept 



up in the cushion by pouring 



mercury into the funnel A, and 



screwing up the cock c, so that 



the stream flowing from it be 



small, and then a portion of the 



air passing through the india- 



rubber will be carried along 



with the mercury. This air may 



be collected in the cylinder B. 



Its composition proves to be 



about 42 volumes of oxygen with 



57 volumes of nitrogen, and one 



volume of carbonic anhydride, 



whilst ordinary air contains 



only 21 volumes of oxygen in 



100 volumes. A square metre of 



india-rubber surface (of the usual 



thickness) passes about 45 c.c. of 



such air per hour. This experi- 



ment clearly shows that india- 



rubber is permeable to gases. 



This may, by the way, be ob- 



served in common toy balloons 



filled with coal-gas. They fall 



after a day or two, not be- 



cause there are holes in them, 



but because air penetrates into, 



and the gas from, their interior, 



through the surface of the india- 



rubber of which they are made. The rate of the passage of gases through india- 



rubber does not, as Mitchell and Graham showed, depend on their densities, and con- 



sequently its permeability is not determined by orifices. It more resembles dialysis 



that is, the penetration of liquids through colloid surfaces. Equal volumes of gases 



penetrate through india-rubber in periods of time which are related to each other as 



follows : carbonic anhydride, 100 ; hydrogen, 247 ; oxygen, 582 ; marsh gas, 688 ; carbonic 



oxide, 1220 ; nitrogen, 1858. Hence nitrogen penetrates more slowly than oxygen, and 



carbonic anhydride more quickly than other gases. 2' 556 volumes of oxygen and 



Fra> 2 7.-Graham's apparatus for the decomposition of air 

 by pumping it through india-rubber. 



