HEAT AND AIR. 165 



grateful to those who made and applied the dis- 

 coveries to which we owe these. And we have 

 this to encourage us in the matter, that the whole is 

 the result of observation of that observation of na- 

 ture which is far more open to us than it was to those 

 men, for they have left us their keys. 



But if the aerial state of things be, as it certainly 

 is, the real and only state in which nature acts, then 

 the atmosphere must necessarily be the general 

 theatre of nature's acting. Nor is there any doubt 

 that it is. There are, indeed, some operations which 

 could not be carried on in the atmosphere, because 

 some of the materials would be dissipated by that ; 

 and there are others in which all the materials would 

 go off together. Thus we can get the water out of 

 brine, and leave the salt, or the spirit out of wash, 

 and leave the water, by boiling in the open air ; but 

 we must be contented to lose the water in the one 

 case, and the spirit in the other. Nor have we any 

 means by which we can, in the open air, and by boil- 

 ing, get out the salt and leave the water, or the water 

 and leave the spirit. In like manner, we may in an 

 open fire drive the charcoal and the bitumen out of 

 common coal, and leave the clay and the iron with 

 which coal is sometimes mixed ; but we cannot, in 

 an open fire, refine the coal by taking out the iron 

 and clay. Every change that we make in the heat 

 of any thing, the atmosphere affects that thing in a 

 different manner ; and it is the same whether the 

 change be produced by nature or by art, or whether 

 it take place in the atmosphere, or in that which is 

 exposed to the atmosphere. Only, we must bear in 

 mind that the atmosphere and the object act differ- 

 ently ; and thus the effect of heating the atmosphere 

 is the same as that of cooling the object, and that 

 of heating the object is the same as cooling the 

 atmosphere. 



The perfect mobility of the atmosphere is one of 

 its most striking and its most useful properties. 



