206 EXTENSIVE USE OF HEAT. 



and every change of appearance that we can ima 

 gj ne for we can rationally imagine nothing but a 

 modification of something we, by the operation of 

 mind, have seen altered in its relation ; and it is no 

 more violation of propriety to suppose that the same 

 heat which keeps the body warm in life, which 

 labours in the furnace and cooks in the fire, and 

 which brings us the beauty of summer and the abun- 

 dance of autumn, can sport in the aurora borealis, 

 guide mariners in the needle, or blaze in the light- 

 ning, than it is to suppose that the never-thawing 

 ice of Mont Blanc could be a river or part of a tree, 

 or a human body, or that its component parts might 

 become the fuel of the most intense flame that is 

 known, and that they are the chief materials in the 

 flames of our common fires. 



At whatever place of the atmosphere water remains 

 in a state of rest, the heat is always such as to bal- 

 ance to the utmost nicety both the gravitation and 

 the cohesion; and it is only the air which at the 

 same point admits of variation. The others are, 

 compared with it, dull and passive properties ; and 

 they act only when heat is suspended, though when 

 once begun, they increase at the rate which has been 

 mentioned ; and, as their action goes on, and water 

 has no solid cohesion at a temperate heat, the result 

 is most conspicuous as gravitation. 



The extreme mobility of the air favours the action 

 of all these principles ; and that is the reason why 

 the aerial state is to be regarded as the elementary 

 state in the formation of all material things. Easily 

 as the air is moveable at the surface of the earth, it 

 must become more and more so as we ascend above 

 that surface, till at its upper limit we can hardly 

 imagine that it offers any resistance at all. We, 

 indeed, know nothing about absolute limits in nature ; 

 but where there ceases to be any resistance is the 

 limit to our observation, and therefore there can be 

 no knowledge, and need be no speculation beyond. 



