110 



of evaporation will therefore about agree with the mean temperature 



of 260\ Thus we can also consider the value of 9.96 kg. cal. as 



the mean lieat of evaporation of the liquid violet phosphorus over the 



temperature range from 512° to 630, so that this heat of evaporation 



will aliout hold for the mean temperature of 571°. Thus we arrive 



at the result that the heat of evaporation from 260° to 571° decreases 



by 2210 gr. cal., so that we have at a rough approximation 



dQ 



J:^ — — 7,106 (4) 



dT ^ ' 



If we now start from the equation : 



^ = JL (5) 



dT RT' ^ ' 



and write : 



Qr= Qo+aT (6) 



we tind by integration: 



••"'=-Tt + ''r'"''+'' '" 



and as according to (6) 



d T ~" 

 we can substitute the value given by (4) for a. Then equation (7) 

 becomes 



Inp = _ ^ _ 3.59 InT + C ') (8) 



^ RT ' 



To see whether this formula satisfied, the following graphical 

 method was applied : Let us write equation (8) as follows : 



rinp -\-Z.h^Th,T=-^ + CT (9) 



R 



we see at once that when this relation satisties, (T/«/; + 3,59 Tin T) 

 plotted as function of T, will have to yield a straight line. 



As appears from fig. 2, the thus obtained points lie really on a 

 straight line ef. so that it has thus been proved that the relation 

 (6) represents the change of the heat of evaporation with the tem- 

 perature with sufficient accuracy. 



In case of an exceptionally rapid heating, when the result was of 

 course less accurate, a pressure of 7,36 atm. was observed at 409°,3, 

 from which the value 1362 follows ïor T Inp. In comparison with the 



1) We may just state here that instead of 3.59 we might as well have taken 

 3 or 4, for the course by which we have come to this value, is a rough approxi- 

 mation. 



