On the Powers of Steam, &c. 117 



to that of the external air, which is allowed never to 

 go to the freezing point, all further escape of heat 

 must cease, and consequently the freezing could never 

 take place. 



With respect to the second supposition, of the 

 cold entering through the pores of the vessel, it must 

 first be remarked, that the consideration of cold, as a 

 positive body or quality, seems to have lost ground 

 greatly. But even admitting this theory, still it is to 

 be remembered, that this air itself is allowed to be 

 always warmer than the freezing point. In endea- 

 vouring to account for this phenomenon, we would 

 say, that the water, transuding through the porous 

 vessels, is converted into vapour ; and, in acquiring 

 that form, absorbs a large quantity of heat from the 

 water in the pans. By a continuance of this process, 

 the water is, at last, robbed of its heat, to such a de- 

 gree as to freeze. 



There is one circumstance, mentioned by Sir Ro- 

 bert, which seems strongly to confirm this supposi- 

 tion. He observes, that the process of making ice 

 will not succeed in changes of winds, and cloudy 

 weather ; and that, in a very sharp, cold night, he has 

 been surprised to find little or no ice formed ; while, 

 on the contrary, in a clear, serene night, with very 

 little dew, though sensibly warmer, the contents of 

 the pan have been frozen into one solid cake. 



It is well known, that evaporation goes on most 

 quickly in a clear dry air ; and that when the atmos- 



