﻿of Phosphorus , Sulphur, and Aldehyde. 517 



curve I. (fig. 2) will show that the velocity of the reaction 

 beginning with the value at 700 millim. increases at first 

 very rapidly as the pressure falls, then varies between narrow 

 limits over a considerable range of pressure (viz., from 500- 

 100 millim.), and finally decreases again rapidly. Curve II. 



(fig. 2) is obtained by dividing the values of -g- from which 



curve I. is constructed by the corresponding values of 



p 

 log p _ , , the rate of evaporation of the phosphorus. That 



is, it represents the rate at which the reaction would go 

 forward if the rate of evaporation of the phosphorus were 

 constant. The numbers so obtained (represented in the figure 

 by circles) evidently lie on a straight line passing through 

 the origin. That is, the corrected velocities are proportional 

 to the partial pressures of the oxygen. This, however, is 

 only true up to a pressure of about 520 millim. At higher 

 pressures the curve changes its direction, and the velocity 

 very quickly decreases to 0. 



A more accurate way of testing the truth of this relation- 

 ship is to be found in the calculation of the values of the 

 constant K in equation 2 b. On looking over the numbers 

 given in Table II., it will be seen that the values of K (at 

 20°) are approximately constant at pressures smaller than 

 550 millim. The numbers sometimes increase, sometimes 

 decrease ; the variations may therefore be ascribed to experi- 

 mental error. 



The same is true for the experiments made at 30°, for 

 pressures between 200 and 25 millim. (no measurements 

 were made at pressures greater than 200 millim.). Below 

 25 millim., however, there is always a marked decrease 

 in the values of K, and traces of a similar behaviour are 

 to be found in the experiments at 20°, and also in the 

 one experiment at 9°. This diminution in the velocity of 

 the reaction at low pressures may possibly have been due 

 to the steam and nitrogen with which the oxygen was mixed 

 hindering the interdiffusion of the oxygen and phosphorus- 

 vapour. 



We may conclude, therefore, that wet oxygen at ordinary 

 temperatures acts on phosphorus with a velocity which may 

 be represented by the equation 



dp v P 



-^ = K. i >.log p — 7 . 



1 



