1899-1900.] Prof. Kuenen on Absorption of a Gas. 
313 
dioxide and hydrogen, 6 we may with safety draw conclusions with 
regard to combinations of water and alcohol with a gas. 
It may here he mentioned that the thermodynamical theory 
of mixtures does not lead to a definite law for the variation of the 
coefficient of absorption with temperature, unless special assump- 
tions are made with regard to the equation of condition of the 
mixture and the constants which it contains. But even without 
doing that, our present knowledge of the behaviour of mixtures 
of the kind indicated above enables us to show the direction in 
which this coefficient will change. 
The coefficient of absorption, as used by Bunsen, represents the 
volume of the gas, reduced to 0°, which is absorbed by unit-volume 
of the liquid. This gas-volume is, by Boyle’s law, independent 
of the pressure as long as Henry’s law holds, i.e ., as long as the 
quantity of gas absorbed is proportional to the partial pressure of 
the gas. This law is in many cases sufficiently correct for low 
pressures, and as long as the temperature is not too high. On the 
other hand, it cannot be true near the critical point of the liquid ; 
the absorption of the gas lowers the critical temperature of the 
liquid, or, to put it more correctly, it gives a mixture whose 
critical point is lower than that of the liquid. The consequence 
is that if the temperature is near the critical point of the liquid, 
the absorption itself may make the liquid disappear, and the law 
of absorption is naturally no longer valid. 
It is easily seen how we have to modify the definition of 
coefficient of absorption so that we may still use it when Henry’s 
law begins to fail. Instead of considering the volume of gas 
absorbed reduced to 0°, or, which comes to the same, the gas- 
volume reduced to 0° and normal pressure, divided by the partial 
pressure of the gas, we must take the limiting ratio of the latter 
quantities for infinitely small absorption. Bor the sake of sim- 
plicity of expression, we may substitute mass for “ volume reduced 
to 0° and normal pressure.” Finally, it will be more convenient 
as well as more natural to consider the mass of gas absorbed by a 
constant mass of liquid instead of by a constant volume of liquid, 
a modification which does not affect appreciably results obtained 
at low temperatures, but will make itself felt as the liquid 
1 Verschaffelt, Communications, Leiden, No. 45. 
