1883.] On the Temperature of Volatilization of Solids. 309 



different pressures, as liquids have definite boiling points, and whether 

 these pressures are identical with their vapour tensions. 



When a liquid is heated by conduction,"^ as for example when a 

 flame is placed below a vessel containing liquid, till the temperature 

 rises to its boiling point, either ebullition or superheating takes place. 

 It would thus seem that, in the case of superheating, the surface of the 

 liquid is not large enough to afford escape for the gaseous molecules 

 liberated ; or the convection currents are not rapid enough to convey 

 the superheated liquid from the lower strata to the surface. If the 

 surface is superheated, the first reason would seem to be correct ; if 

 the surface, on the other hand, is at its normal temperature, the second 

 explanation is applicable. When ebullition takes place, the liquid 

 increases its surface by the formation of bubbles, and as heat is totally 

 absorbed in producing evaporation, the temperature of the liquid 

 remains constant. If the supply of heat is increased, evaporation is 

 also more rapid, and facility is given to more rapid evaporation, by the 

 formation of more numerous and larger bubbles, which increase the 

 evaporating surface. 



A solid has a limited surface, which, cannot be increased by forma- 

 tion of bubbles ; and it might therefore be expected that on increasing 

 the supply of heat, the temperature of the^solid should rise, until a 

 temperature is reached at which its rate of evaporation is equivalent 

 to the rate at which heat is commuuicated to it. Reasoning thus, the 

 existence of hot ice was maintained by Dr. Carnelley and other writers 

 in a series of letters which appeared in " Nature " during the years 

 1881 and 1882. 



On the other hand, a liquid in the spheroidal state presents a free 

 surface of evaporation in every direction, and yet although exposed to 

 radiation from a white-hot surface, its temperature does not rise even 

 to the boiling point ; and we find it impossible to raise the temperature 

 of water above 90°, by applying heat to its surface. In such cases 

 the surface appears to be large enough to allow vapour to escape 

 with sufficient rapidity to prevent superheating. ' 



If, then, the rate of evaporation at the surface of a solid is indepen- 

 dent of the extent of that surface, but is influenced only by the rate 

 at which heat is communicated to it, and, as in the case of liquids, by 

 the pressure to which it is exposed, it follows that solids have definite 

 temperatures of evaporation, or subliming points, corresponding to 

 definite pressures, as liquids have definite boiling points. 



Our experiments with water, acetic acid, benzene, naphthalene, and 

 camphor, show that this is the case ; and that with ice and camphor 

 these pressures are sensibly the same as the maximum tensions of the 

 vapours of these solids at corresponding temperatures. In the case of 

 ice, the maximum temperature which, can be attained under any given 

 pressure is indicated by James Thomson's ice-steam line. 



