HEAT AND AIR. 165 



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 

 boiling, 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 differently ; 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. 

 We are not authorized to say that it moves with- 

 out any friction ; but its friction is only the fric- 

 tion of particles ; and, with moderate velocities, 

 the resistance of air rubbing on air is very small. 



The atmospheric air is at once the most delicate 

 and the most powerful of all springs. It actually 

 yields to the touch of a sunbeam, and yet it can 

 cleave rocks, and shake the surfaces of countries to 

 pieces in earthquakes. It is more nice in the 

 detection of pressure than any instrument that we 



