CHAP, 1. 10. OF CLOUDS. 33 



on without a cirrose crown on the upper part 

 of the nimbus. I have noticed the spon- 



new form, and becomes a distinct elastic fluid, called steam: 

 whether water first chemically combined with air, and then, 

 heated above 212, is detached from the air or remains with 

 it, the advocates of the theory have not determined. This 

 theory has always been considered as complex, and attended 

 with difficulties: so much that M. Pictet and others have 

 rejected it, and adopted that which admits of distinct elastic 

 vapours in the atmosphere at all temperatures, uncombined 

 with either of the principal constituent gases, as being much 

 more simple and easy of explication than the other; though 

 they do not remove the grand objection to it, arising from 

 atmospheric pressure." 



On the Evaporation of Water below 212. 



" I have frequently tried the evaporation at all the tem- 

 peratures below 212: it would be tedious to enter into detail 

 of all the experiments, but I shall give the results at some 

 remarkable points. In all the high temperatures I used the 

 vessel abovementioned,* keeping a thermometer in it, by 

 which I could secure a constant heat, or at least keep it 

 oscillated within narrow limits. 



" The evaporation from water of 180 was from 18 to 22 

 grains per minute, according to circumstances; or about one 

 half of that at 212. 



" At 164 it was about one third of the quantity at the 

 boiling temperature, or from 10 to 16 grains per minute. 



* This refers to experiments on the evaporation of water at 212, for 

 which see the Essay. 



