82 PRINCIPLES OF CHEMISTRY 



then water would dissolve so much of each of these gases as would be 

 dissolved if each separately exerted a pressure of half an atmosphere, 

 and in this case, at one cubic centimetre of water would dissolve 

 0-02 cubic centimetre of oxygen and 0*90 cubic centimetre of carbonic 

 anhydride. If the pressure of a gaseous mixture equals It, and in u 

 volumes of the mixture there be a volumes of a given gas, then its 

 solution will proceed as though this gas were dissolved under a pres- 

 sure - . That portion of the pressure under influence of which the 



solution proceeds is termed the ' partial ' pressure. 



In order to represent to oneself the cause of the law of partial 

 pressures, an explanation must be given of the fundamental properties 

 of gases. Gases are elastic and disperse in all directions. All that is 

 known of gases obliges one to think that these fundamental properties 

 of gases are due to a rapid progressive movement, in all directions, 

 which is proper to their smallest particles (molecules). 35 These mole- 

 cules in impinging against an obstacle produce a pressure. The greater 

 the number of molecules impinging against an obstacle in a given time, 

 the greater the pressure. The pressure of a separate gas or of a gaseous 

 mixture depends on the sum of the pressures of all the molecules, on 

 the number of blows in a unit of time on a unit of surface, and on the 

 mass and velocity (or the vis viva) of the impinging molecules. To the 

 obstacle all molecules (although different in nature) are alike ; it is 

 submitted to a pressure due to the sum of their vis viva. But, in a 

 chemical action such as the solution of gases, on the contrary, the 



50 Although the actual movement of gaseous molecules, which is acknowledged by the 

 kinetic theory of gases, cannot be seen, yet its existence may be rendered evident by 

 taking advantage of the difference in the velocities which undoubtedly belongs to 

 different gases which are of different densities under equal pressures. The molecules of a 

 light gas must move more rapidly than the molecules of a heavier gas in order to produce 

 the same pressure. Let us take, therefore, two gases hydrogen and air ; the former is 

 14'4 times lighter than the latter, and hence the molecules of hydrogen must move almost 

 four times more quickly than air (more exactly 3'8, according to the formula given in the 

 preceding footnote). Consequently, if air occurs inside a porous cylinder and hydrogen 

 outside, then in a given time the volume of hydrogen which succeeds in entering the 

 cylinder will be greater than the volume of air leaving the cylinder, and therefore the 

 pressure inside the cylinder will rise until the gaseous mixture (of air and hydrogen) 

 attains an equal density both inside and outside the cylinder. If now the experiment 

 be reversed and air surround the cylinder, and hydrogen be inside the cylinder, then more 

 gas will leave the cylinder than enters it, and hence the pressure inside the cylinder 

 will be diminished. In these considerations we have replaced the idea of the number 

 of molecules by the idea of volumes. We shall learn afterwards that equal volumes 

 of different gases contain an equal number of molecules (the law of Avogadro-Ger- 

 hardt), and therefore instead of speaking of the number of molecules we can speak of 

 the number of volumes. If the cylinder be partially immersed in water the rise and fall 

 of the pressure can be observed, and consequently the experiment can be rendered self- 

 evident. 



