202 HELMHOLTZ 



dependency of the direction of the wind, and, successfully 

 resisting storms which would drive 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 intelligent 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 de- 

 velopment 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 force 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 no-v with Heat in this respect? 



This question was of decisive importance in the endeavour 

 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 

 Mechanical Theory of Heat. 



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

 stance, very fine and imponderable indeed, but indestructible, 

 and unchangeable in quantity, which is an essential funda- 

 mental 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. 



By conduction and radiation, it can indeed pass from 

 hotter to colder bodies; but the quantity of heat which the 

 former lose can be shown by the thermometer to have re- 

 appeared in the latter. Many processes, too, were known, 

 especially in the passage of bodies from the solid to the 

 liquid and gaseous states, in which heat disappeared at any 

 rate, as regards the thermometer. But when the gaseous 



