342 01^ THE CONSERVATION OF FORCE. 



sailing-vessels far away, reach their goal at the appointed 

 time. The advantages which the concourse of numerous, 

 and variously skilled workmen in all branches offers in 

 large towns where wind and water power are wanting, can 

 be utilised, for steam-engines find place everywhere, 

 and supply the necessary crude force ; thus the more in- 

 telligent human force may be spared for better purposes ; 

 and, indeed, wherever the nature of the ground or the 

 neighbourhood of suitable lines of communication present 

 a favourable opportunity for the development of industry, 

 the motive power is also present in the form of steam- 

 engines. 



We see, then, that heat can produce mechanical power ; 

 but in the cases which we have discussed we have seen 

 that the quantity of farce which can be produced by a 

 given measure of a physical process is always accurately 

 defined, and that the further capacity for work of the 

 natural forces, is either diminished or exhausted by the 

 work which has been performed. How is it now with Heat 

 in this respect ? 



This question was of decisive importance in the en- 

 deavour to extend the law of the Conservation of Force 

 to all natural processes. In the answer lay the chief 

 difference between the older and newer views in these 

 respects. Hence it is that many physicists designate 

 that view of Nature corresponding to the law of the 

 conservation of force with the name of the Mechanical 

 Theoi^ of Heat. 



The older view of the nature of heat was that it is a 

 substance, very fine and imponderable indeed, but in- 

 destructible, and unchangeable in quantity, which is an 

 essential fundamental property of all matter. And, in 

 fact, in a large number of natural processes, the quantity 

 of heat which can be demonstrated by the thermometer 

 is unchangeable. 



