DIFFUSION OF GASES. 75 



perature of 12 C. and a pressure of 760 mm. of mercury, absorbs 35 volumes of 

 carbon dioxid, 9.4 volumes of oxygen, 7.5 volumes of nitrogen, 1.5 volumes of 

 hydrogen. The absorption of the gases is invariably attended with the generation 

 of heat, which is in proportion to the energy with which absorption takes place. 

 Non-porous bodies are in an analogous manner surrounded intimately upon their 

 surface by a layer of condensed gas. 



Fluids are in like manner capable of taking up or absorbing gases. In this 

 connection it has been learned that a given amount of fluid at different pressures 

 nevertheless always absorbs an equal volume of gas. Whether the pressure be 

 great or small, the volume of gas absorbed is always the same. It is, however, 

 known, according to the law of Boyle-Mariotte, governing the compression of gases, 

 that with twice, thrice or greater amounts of pressure, twice, thrice or greater 

 amounts of gas by weight are contained within an equal volume of gas. From 

 this there is formulated the law that while at varying pressures the volume of 

 gas absorbed remains the same, the amount of gas by weight contained within 

 the same volume is directly proportional to the amount' of pressure. If, therefore, 

 the pressure is zero the amount of the absorbed gas must likewise be zero; 

 whence it follows that fluids under the air-pump in a vacuum may be deprived of 

 their absorbed gases. 



The coefficient o absorption represents that volume of gas that is absorbed 

 by i volume-unit of a fluid at a given pressure and temperature. From what 

 has been said with regard to the volume of absorbed gases the coefficient of ab- 

 sorption must be wholly independent of the pressure. 



The temperature has an important influence upon the coefficient of absorption. 

 When the temperature is low the coefficient is highest, declining at a higher tem- 

 perature and becoming zero when the fluid boils. From this it follows that ab- 

 sorbed gases can also be expelled from fluids by heating the latter to the boiling- 

 point. The coefficient of absorption increases, however, for various fluids and 

 gases with increasing temperature in a peculiar, and by no means uniform, manner, 

 which must be determined empirically for each. At the temperature of the body 

 the coefficient of absorption of carbon dioxid is 0.5283, of nitrogen 0.0119, of oxy- 

 gen, at a pressure of 699 mm., 0.0231. 



DIFFUSION OF GASES; ABSORPTION OF GASEOUS MIXTURES. 



Gases that do not enter into chemical combination with one another are 

 capable of forming a uniform mixture. If, for instance, the necks of two flasks 

 are connected of which the lower contains carbon dioxid and the upper, placed 

 vertically and inverted above the other, contains hydrogen, both gases combine, 

 independently of differences in specific gravity, within each flask so as to form 

 identical mixtures. This phenomenon is known as the diffusion of gases. If a 

 porous membrane be previously interposed between the two gases the interchange 

 of gases takes place just the same. Nevertheless different gases pass through the 

 interstices of the membrane with unequal rapidity in the same way as in the 

 case of fluids in the process of endosmosis, so that at first a larger amount of gas 

 will be present upon the one side than upon the other. According to Graham 

 the rapidity with which gases pass through the interstices is inversely as the 

 square root of their specific gravity, but according to Bunsen, not exactly so. 



Gases mutually exert no pressure upon one another. Therefore a gas escapes 

 from a space containing another gas as from a vacuum. If, accordingly, the sur- 

 face of a fluid in which a gas is absorbed be placed in communication with a large 

 amount of another gas, the absorbed gas passes over into the other gas. Therefore, 

 absorbed gases can be removed if the fluids containing them are treated with 

 other gases by agitation or by passing them through. 



If two or more gases in mixture lie over a fluid within a closed space the separate 

 gases will be absorbed, and according to weight in proportion to the pressure to 

 which each gas would be exposed if it were alone present in the space. This 

 pressure is known as partial pressure. The amount of gas absorbed from mixtures 

 is therefore proportionate to the partial pressure. The partial pressure of a gas 

 in a space partially filled by a fluid is at the same time an expression of the ten- 

 sion of the absorbed gas in this fluid. 



The air contains 0.2096 volume of oxygen and 0.7904 volume of nitrogen. 

 If, therefore, one volume of air is present at a pressure P over water, the partial 

 pressure under which oxygen is absorbed is 0.2096 x P, and that for nitrogen 

 equals 0.7904 x P. At a temperature of o C. and at 760 mm. pressure i volume 



