8 ROYAL SOCIETY OF CANADA 



at temperatures below zero, breaking down to ordinary wat-er vapour with 

 the absorption of 80 calories of heat per gram. 



The result gives us some insight into the mechanism of sublimation. 

 It shows that the molecules liberated from the solid are of the same com- 

 plexity as the solid molecule, and for a short space of time, probably only 

 a fraction of a second, exist as a solid vapour phase. It shows us fur- 

 ther that the evaporation of ice is directly into a vapour without passing 

 through a preliminary liquid phase. 



TABLE IV. 

 Heat of Sublimation. 



No attempt has yet been made to find the variation in the heat of 

 sublimation of ice with temperature, but from theoretical reasoning it 

 is unlikely that this changes very much. 



Specific Heat of Ice. 



Of the earlier investigators who turned their attention to a deter- 

 mination of the specific heat of ice, we may pass over the work of Des- 

 ormes and Clement and of Avogadro. Thus the former obtained the 

 values 0.75 and 0.70 and the latter 0.92 by somewhat similar methods. 

 The two general methods used in the later work are, in one case, the 

 miving of ice cooled below the freezing point with warm water, and after 

 correcting for the heat of fusion to estimate the specific heat. The 

 other is to mix cooled ice with oil of turpentine and arrange that the 

 minimum temperature of the mixture shall not be above the melting 

 point. By this means a knowledge of the heat of fusion is avoided, but 

 on the other hand, an accurate knowledge of the specific heat of tur- 

 pentine at difl'erent temperatures is required. 



In 1849 Regnault determined the specific heat between — 78° and 

 0°C., obtaining the value 0.474. In 1843 Desains made a similar mea- 

 surement between — 20° and 0°C., obtaining 0.513. Another value 

 obtained by him is given as 0.47, from which he concludes that the 

 correct number is 0.51. 



