﻿of Phosphorus, Sulphur, and Aldehyde. 50 ( J 



Influence of the Rate of Evaporation. 



The fact that K 7 increases as the pressure falls shows that 

 the reaction takes place somewhat faster at lower pressures 

 than one would expect if its velocity were directly propor- 

 tional to the pressure. This may be due to the fact that 

 substances evaporate more rapidly into a gaseous atmosphere 

 when its pressure is small than when it is larger. There can 

 be little doubt that the reaction takes place between phos- 

 phorus-vapour and oxygen ; and, furthermore, if it is allow- 

 able to take the phosphorescent light as an indication of the 

 locality of the reaction, it takes place close to the surface of 

 the phosphorus so long as the pressure of the oxygen is not 

 very small*. There are thus grounds for supposing that the 

 phosphorus-vapour is oxidized as fast as it is evolved from 

 the surface of the phosphorus, and therefore that the velocity 

 with which the reaction goes forward will be directly pro- 

 portional to the rate of evaporation of the phosphorus. By 

 making this assumption, as we shall see, it is at any rate 

 possible to give a coherent and fairly satisfactory account of 

 reactions the courses of which would otherwise appear to be 

 totally exceptional. 



According to Stefan f, when all the other conditions remain 

 constant, the rate of evaporation of a liquid into a gas is 

 connected with the pressure of the latter by the following 

 formula : — 



, P 



where r " 



v = the rate of evaporation, 

 P= total pressure of the gas and vapour, 

 p f = the vapour-pressure of the liquid, 



and c = some constant. 



Introducing this correction for the changeable velocity of 

 evaporation into equation (1), we obtain 



-|=%->ogp^ (2) 



In order to integrate this equation we may w r rite p = F— a, 

 where a is the partial pressure of the nitrogen and aqueous 

 vapour in the mixture of gases, and therefore constant. 

 After making this substitution and expanding the logarithm, 



* The luminous phenomena accompanying the oxidation of phosphorus 

 have been carefully described b\ v. Marum, loo. cit. ; Joubert, Theses, 

 1874; Fischer, J. prakt. Chem' xxxv. p. 343(1845); Schrbtter, Sitz- 

 ungsber. Wien. Akad, ix. p. 414 (1852), and others. 



t Sitzungsber. K. Akad. d. Wiss. Wien, lxviii. 1878, p. 385. 



