286 Mr. S. Skinner. [Apr. 21. 



Fig. 1. 



my own observations, and wonld run alongside of that for HC1 if 

 continued below zero. The boiling point of PH 3 is given by 

 Olszewski as —85°. 



At first sight it is evident that the form of the curve for PH 4 C1 is 

 not a normal one, for if ifc were it would lie in the same direction as 

 the other two. At all temperatures the pressure is much lower than 

 that corresponding to a mixture of the two gases PH 3 and HC1, so 

 that it is evident forces of chemical attraction are acting. From 

 — 30° to 10° the curve runs normally, so it appears that the gas 

 probably consists wholly, from reasons we shall presently point out, of 

 PH 3 and HC1 molecules. Above 10° these two gases combine and the 

 pressure necessary to deposit the crystals very rapidly increases. If 

 the temperature be maintained constant the whole of the mixture is 

 converted to the crystals of the compound under constant pressure by 

 reducing the volume, just as a saturated vapour is converted into a 

 liquid under similar conditions. 



At the temperature at which the crystals melt there should be an 



