ON WATKII AND ITS COMPOUNDS 83 



nature of the impinging molecules plays the most important part. In 

 impinging against a liquid, a portion of the gas enters into the liquid 

 itself, and is held by it so long as other gaseous molecules impinge 

 against the liquid exert a pressure on it. As regards the solubility of 

 a given gas, for the number of blows it makes on the surface of a liquid, 

 it is immaterial whether other molecules of gases impinge side by side 

 with it or not. Therefore, the solubility of a given gas will be propor- 

 tional, not to the total pressure of a gaseous mixture, but to that por- 

 tion of it which is due to the given gas separately. Further, the satura- 

 tion of a liquid by a gas depends on the fact that the molecules of 

 gases that have entered into a liquid do not remain at rest in it, 

 although they enter in a harmonious kind of movement with the mole- 

 cules of the liquid, and therefore they throw themselves off from the 

 surface of the liquid (just like its vapour if the liquid be volatile). If 

 in a unit of time an equal number of molecules penetrate into (leap 

 into) a liquid and leave (or leap out of) a liquid, it is saturated. It 

 is a case of mobile equilibrium, and not of rest. Therefore, if the 

 pressure be diminished, the number of molecules departing from the 

 liquid will exceed the number of molecules entering into the liquid, 

 and a fresh state of mobile equilibrium only takes place under a fresh 

 equality of the number of molecules departing from and entering into 

 the liquid. Thus are explained the main features of the solution, and 

 furthermore of that special (chemical) attraction (penetration and har- 

 monious movement) of a gas for a liquid, which determines both the 

 measure of solubility and the degree of stability of the solutions pro- 

 duced. 



The consequences of the law of partial pressures are exceedingly 

 numerous and important. All liquids in nature are in contact with the 

 atmosphere. The atmosphere, as we shall afterwards see more fully, 

 consists of an intermixture of gases, chiefly four in number oxygen, 

 nitrogen, carbonic anhydride, and aqueous vapour. 100 volumes of 

 air contain, approximately, 78 volumes of nitrogen, and about 21 

 volumes of oxygen ; the quantity of carbonic anhydride, by volume, 

 does not exceed 0'05. Under ordinary circumstances, the quantity of 

 aqueous vapour is much greater, but it varies with the moisture of the 

 atmosphere. Consequently, the solution of nitrogen in a liquid in 

 contact with the atmosphere will proceed under a partial pressure equal 

 to j 7 ( * x 760 mm. if the atmospheric pressure equal 760 mm. ; con- 

 sequently, under a pressure of 600 mm. of mercury, the solution of 

 oxygen will proceed under a partial pressure of about 160 mm., and 

 the solution of carbonic anhydride only under the very small pressure 

 of 0'4 mm. Therefore, although the amount of nitrogen in air is 



G2 



