William Thomson 29 



first to the second, transform part of the thermal energy 

 into work, but only a certain definite portion, exactly 

 calculable in accordance with the second law and Carnot's 

 principle. But if we bring the hot and cold bodies into 

 actual contact with each other, and allow the heat to pass 

 directly from one to the other, without doing mechanical 

 work, their temperature will be equalized, and we shall have 

 lost for ever the possibility of utilizing the thermal energy 

 which has been transferred. There is, therefore, a funda- 

 mental difference between the transformation of mechanical 

 work into heat and the inverse transformation. In the 

 former case we may convert the whole mechanical energy 

 into heat, as when we rub two bodies together and raise 

 their temperature through friction, while, in the reverse 

 operation, when heat is transformed into work, only part 

 of that which leaves the source of heat is utilized. We must 

 therefore distinguish in the energy of a body a part which 

 is available for the performance of useful work, and another 

 part which is unavailable, the thermal energy of a body 

 containing only a definite proportion belonging to the first 

 category. Moreover, it is only the ideally perfect engine 

 that can utilize the whole of the available energy; in 

 machines such as those we can construct there is always a 

 further loss due to their imperfection. We must conclude 

 that in the constantly occurring processes in which heat is 

 allowed to pass from one piece of matter to another without 

 doing useful work, the quantity of available energy stored 

 in the universe is diminished. This leads us to the counter- 

 part of the principle of conservation, which is that of the 

 dissipation of energy. Among the wealth of achievements 

 contained in the intellectual heritage left us by Kelvin, 

 the discovery of this truth is pre-eminently the one which 

 stands out as a landmark to future generations. It was 

 first announced in 1852, and we may quote the main 

 conclusions as then formulated. 



1. There is at present in the material world a universal 

 tendency to the dissipation of mechanical energy. 



2. Any restoration of mechanical energy, without more 

 than an equivalent dissipation, is impossible in inanimate 

 material processes, and is probably never effected by means 



