﻿520 



Dr. T. Ewan on the Rate of Oxidation 



out from the pressure of 200 millim., at which the velocity of 

 the reaction is 0, it appears to increase continuously as the 

 pressure falls, and not to reach a maximum value as is the 

 case when the gas is moist. Curve II. (fig. 3) is obtained by 

 dividing the values of the rate of reaction from which curve I. 

 is drawn by the corresponding values of the rate of evapora- 

 tion of the phosphorus, and plotting the numbers so obtained 

 against the pressures. The numbers are so irregular that it 

 is not easy to make out the true nature of the curve. The 

 values of the ordinates of the part of the curve between and 

 70 millim., however, appear to be proportional to the square 

 roots of the corresponding values of the pressure. 

 We have, accordingly, 



J^^logj^,, 



• • (3) 



which is the same equation as has already been found to hold 

 good for phosphorus and moist oxygen, except that the velo- 

 city of the reaction is put proportional to the square root of 

 the partial pressure of the oxygen instead of to the pressure 

 itself. 



This equation may be integrated by expanding the logarithm 

 in the same way as before : this gives, neglecting small 

 terms, 



The value of the constant is obtained from the condition 

 that P = P , when £=0 ; P being the total pressure at the 

 beginning of the experiment. Introducing the value of the 

 constant, we obtain 



Ki<=3^[(Po-«) § -(P-a)']+(|-l)[(P -a)*-(P-a)i].(3a 



It is by means of this expression that the values of K x given 

 in Table III. have been calculated. The numbers show that 

 Kj is approximately constant from a pressure of 60-70 millim. 

 downwards ; at higher pressures it diminishes. The irregu- 

 larity of the numbers is probably due, in part at any rate, to 

 the deposition of a coating of oxide on the surface of the 

 phosphorus. We may therefore say that the rate of eva- 

 poration of the phosphorus being supposed constant, it is 

 acted on by dry oxygen with a velocity which is proportional 

 to the square root of its pressure. This is only true (at 20°) 

 up to a pressure of some 60-70 millim. ; above this pressure 

 the velocity decreases. It is of interest that the greatest 



