288 PHYSIOLOGICAL PHYSICS. [Chap. xxm. 



oxygen has been liquefied by a pressure of 300 

 atmospheres, aided by a very low temperature, 

 obtained by the evaporation of liquid sulphurous 

 acid and solid carbonic acid, and other means. 

 Nitrogen required a pressure of 200, and hydrogen 

 of 280 atmospheres. 



DIFFUSION AND ABSORPTION OF GASES. 



Diffusion of gases. When two gases are 

 placed in contact with one another at the same tem- 

 perature and pressure, they mix rapidly until the one 

 gas is uniformly diffused throughout the other. The 

 diffusion is quite independent of gravity, for it will 

 occur between a mass of carbonic acid gas below and 



O 



a mass of hydrogen above, the heavy gas rising up 

 into the light one, and the light one diffusing through- 

 out the heavy one below. All gases possess this 

 property in virtue of their tendency always to expand 

 and fill any space open to them. One gas will not 

 expand into a space occupied by the same gas, if the 

 temperatures and pressures are the same. But when 

 the gases are different diffusion goes on just as if the 

 gases were expanding into a vacuum, only with 

 diminished speed. In a mixture, according to D ALTON'S 

 LAW, each gas exerts its own pressure as if it were the 

 only gas present, a pressure dependent upon its 

 volume ; and thus the total pressure exerted by the 

 mixed gases will be the sum of the pressures due to 

 each gas separately. The pressure exerted by each 

 gas is called the PARTIAL PRESSURE of each gas in the 

 mixture, and its amount is calculated by multiplying 

 the total pressure by the number representing the 

 amount of gas in 100 volumes of the mixture. Thus, 

 oxygen being present in the atmosphere to the extent, 

 roughly, of 21 volumes in 100, and the atmosphere 

 being at 760mm. pressure, the partial pressure of 

 O is 760 x AV 



