500 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



UJ 



Q. 



apparatus, are given in Table XVIII. and Figure 19. The change is 

 constant so far as could be judged from the points themselves, but the 

 best values at the triple points would demand a slight decrease of this 



volume with decreasing temper- 

 ature. In addition to the four 

 points listed above, there is a 

 fifth determination on April 20 

 by the method described on p. 

 495. This is higher by 4 per 

 cent (0. 2 per cent of the original 

 volume) than the previous point. 

 In view of the fact that the first 

 four points very approximately 

 satisfy the requirements at the 

 triple points, there seems little 

 doubt that they are the correct 

 ones. 



The values of ^H and ^E 

 calculated from these data are 



< 



C9 



-0.83 



-0.89 



-35" 



-30 -25 



TEMPERATURE 



-20" 



Figure 20. The latent heat and 

 the change of internal energy when 

 III passes to V. 



given in Table XIX. and Figure 20. Ill gives out heat on passing 

 into V and absorbs work. The heat is very small, much less than 

 the work, resulting in an increase of internal energy on passing from 

 III to V. 



The Curve II-V. 



This curve has some of the properties of the II-III curve. It is 

 characterized by small change of volume and by the fact that II passes 

 into V immediately when the temperature is raised beyond the curve. 

 The method of determining the points was the same as that of the 

 II-III points. This was the last of all the equilibrium curves to be 

 obtained. It was found after sufficient skill had been attained with 

 the method, so that it was necessary to obtain the points only once. 

 For this reason, the observation that it was impossible- to heat II into the 

 region of V was not tested by so many trials as the corresponding change 

 II-III, and has therefore not quite so much probability of being abso- 

 lutely true. The reaction velocity on this curve drops very much faster 

 than it does on the II-III curve with decreasing temperature. This 

 might, at lower temperatures, result in the apparent possibility of 

 bringing II into the region of V, even if the reaction were actually 

 running. This slowness of reaction makes very difficult the determi- 

 nation of the equilibrium points at the low temperatures, the deter- 

 mination being difficult enough anyway, because of the small change 



