80 PRINCIPLES OF CHEMISTRY 



sure increased 10 times, the solubility of ammonia only increased 4!, 

 times. 



A number of examples of such cases of the absorption of gases 

 by liquids might be cited which do not in any \\;iv, even approximately, 

 agree with the laws of solubility. Thus, for instance, carbonic anhy- 

 dride is absorbed by a solution of caustic potash in water, and if there 

 be sufficient caustic potash it is not separated from the solution by a 

 decrease of pressure. This is a case of more intimate chemical com- 

 bination. A less completely studied, but similar and clearly chemical, 

 correlation appears in certain cases of the solution of gases by water, 

 and we shall afterwards take an example of this in the solution of 

 hydrogen iodide ; but first we will stop to consider a remarkable appli- 

 cation of the law of Henry and Dalton 33 in the case of the solution of 

 a mixture of two gases, and this we must do all the more because the 

 phenomena which then take place cannot be foreseen without a clear 

 theoretical representation of the nature of gases. 34 



53 The ratio between the pressure and the amount of gas dissolved was discovered by 

 Henry in 1805, and Dalton in 1807 pointed out the adaptability of this law to cases of 

 gaseous mixtures, introducing the conception of partial pressures which is absolutely 

 necessary for a right comprehension of Dalton's law. The conception of partial pressures 

 essentially enters into that of the diffusion of vapours in gases (footnote 1) ; for the 

 pressure of damp air is equal to the sum of the pressures of dry air and of the aqueous 

 vapour in it, and it is admitted as a sequence to Dalton's law that evaporation in dry 

 air takes place as in a vacuum. It is, however, necessary to remark that the volume of 

 a mixture of two gases (or vapours) is only approximately equal to the sum of the volumes 

 of its constituents (the same, naturally, also refers to their pressures) that is to say, in 

 mixing gases a change of volume occurs, which, although small, is quite apparent when 

 carefully measured. For instance, in 1888 Brown showed that on mixing various volumes 

 of sulphurous anhydride (SO 2 ) with carbonic anhydride (at equal pressures of 7<>0 mm. 

 and equal temperatures) a decrease of pressure of 3'9 millimetres of mercury was 

 observed The possibility of a chemical action in similar mixtures is evident from the 

 fact that equal volumes of sulphurous and carbonic anhydrides at 19 form, according 

 to Pictet's researches in 1888, a liquid having the signs of a chemical compound, or a 

 solution similar to that given when sulphurous anhydride and water combine into an 

 unstable chemical whole. 



51 The origin of the now generally-accepted kinetic theory of gases, according to 

 which they are animated by a rapid progressive movement, is very ancient (Bernoulli and 

 others in the last century had already developed a similar representation), but it was 

 only generally accepted after the mechanical theory of heat had been established, and 

 after the work of Krb'nig (1855), and especially after its mathematical side had been 

 worked out by Clausius and Maxwell. The pressure, elasticity, diffusion, and internal 

 friction of gases, the laws of Boyle, Mariotte, and of Gay-Lussac and Avogadro-Gerhardt 

 are not only explained (deduced) by the kinetic theory of gases, but also expressed with 

 perfect exactitude ; thus, for example, the magnitude of the internal friction of different 

 gases was foretold with exactitude by Maxwell, by applying the theory of probabilities to 

 the concussion of gaseous particles. The kinetic theory of ga^es must therefore be con- 

 sidered as one of the most brilliant acquisitions of the latter half of the present century. 

 The velocity of the progressive movement of the gaseous particles of a gas, one cubic 

 centimetre of which weighs d grams, is found, according to the theory, to be equal to 

 the square root of the product of SpDg divided by d, where p is the pre>suiv under which 



