oN WATER AND ITS COMPOUNDS 81 



Tin l<nr of partial pressures is as follows : The solubility of gases 

 in intermixture with each other does not depend on the influence of 

 the total pressure acting on the mixture, but on the influence of that 

 portion of the total pressure which is due to the volume of each given gas 

 in the mixture. Thus, for instance, if oxygen and carbonic anhydride 

 were mixed in equal volumes and exerted a pressure of 760 millimetres, 



(1 is determined expressed in centimetres of the mercury column, D the weight of a cubic 

 centimetre of mercury in grams (-0 = 13*59,^ = 76, consequently the normal pressure = 

 l.o:i:; grams on a sq. c. m.), and g the acceleration of gravity in centimetres (^ = 980'5, 

 at the sea level and long. 45, = 981'92 at St. Petersburg ; in general it varies with the 

 longitude and altitude of the locality). Therefore, at the velocity of hydrogen is 1,843, 

 and of oxygen 461, metres per second. This is the average velocity, and (according to- 

 Maxwell and others) it is probable that the velocities of individual particles are different, 

 that is, they occur in, as it were, different conditions of temperature, which is very im- 

 portant to take into consideration in the investigation of many phenomena proper to- 

 matter. It is evident from the above determination of the velocity of gases, that 

 different gases at the same temperature and pressure have average velocities, which are 

 inversely proportional to the square roots of their densities ; this is also shown by direct 

 experiment on the flow of gases through a fine orifice, or through a porous wall. This 

 <l/Nfii>iitt<ii- n -J ncit if of flow for different gases is frequently taken advantage of in 

 chemical researches (see Chap. II. and also Chap. VII. on the law of Avogadro-Gerhardt) 

 in order to separate two gases having different densities and velocities. The difference 

 of the velocity of flow of gases also determines the phenomenon cited in the following 

 footnote for demonstrating the existence of an internal movement in gases. 



If for a certain mass of a gas which fully and exactly follows the laws of Mariotte 

 and Gay-Lussac the temperature t and the pressure p be simultaneously changed, then 

 the entire change would be expressed by the equation pr = C (1 + at), or, what is the 

 same, pv = RT, where T-t + 273 and C and R are constants which vary not only with the 

 units of measurement but with the nature of the gas and its mass. But as there are 

 discrepancies from both the fundamental laws of gases (which will be spoken of in the 

 following chapter), and as, on the one hand, a certain attraction between the gaseous 

 molecules must be admitted, and on the other hand it must be acknowledged that the 

 gaseous molecules themselves occupy a portion of a space, therefore for ordinary gases,, 

 within any considerable variation of pressure and temperature, recourse should be had 

 to Van der Waal's formula 



(p + ^r) (vp) = R (I at) 



where a is the true co-efficient of expansion of gases. As the actual co-efficient of ex- 

 pansion of air at the atmospheric pressure and between temperatures of and 100 = 

 0'00367, when determined from the change of pressure (according to Kegnault's data) 

 and when determined from the change of volume = 0'00368 (according to Mendeleeff and 

 Kayander), and for other gases there is a discrepancy, although not a large one (see the 

 following chapter), which is considerable at high pressures and for great densities, there- 

 fore that co-efficient of expansion should be taken which all gases have at low pressures. 

 This quantity is approximately 0'00367. 



The formula of Van der Waal has an especially important significance in the case 

 of the passage of a gas into a liquid state, because the fundamental properties of both 

 pi M-. and liquids are equally well expressed, although only in their general features, 

 by it. 



The further development of the questions referring to the subjects here touched on, 

 which are of especial interest for the theories of solutions, must be looked for in special 

 memoirs and works on theoretical and physical chemistry. A small part of this subject 

 will be partially considered in the footnotes of the following chapter. 

 VOL. I. 



