THE RECENT PROGRESS OF PHYSICAL SCIENCE. 747 



them by a growing science, and I think that the almost unavoidable 

 use of the word equivalent, in the statement of the first law, is partly- 

 responsible for the little attention that is given to the second. For 

 the second law so far contradicts the usual statement of the first as 

 to assert that equivalents of heat and work are not of equal value. 

 While work can always be converted into heat, heat can only be con- 

 verted into work under certain limitations. For every practical pur- 

 pose the work is worth the most, and when we speak of equivalents 

 we use the word in the same sort of special sense as that in which 

 chemists speak of equivalents of gold and iron. The second law 

 teaches us that the real value of heat, as a source of mechanical 

 power, depends upon the temperature of the body in which it resides ; 

 the hotter the body in relation to its surroundings, the more available 

 the heat. 



In order to see the relations which obtain between the first and 

 the second law of thermo-dynamics, it is only necessary for us to 

 glance at the theory of the steam-engine. Not many years ago calcu- 

 lations were plentiful, demonstrating the inefiiciency of the steam- 

 engine on the basis of a comparison of the work actually got out of 

 the engine with the mechanical equivalent of the heat supplied to 

 the boiler. Such calculations took into account only the first law of 

 thermo-dynamics, which deals with the equivalents of heat and work, 

 and have very little bearing upon the practical question of efficiency, 

 which requires us to have regard also to the second law. According 

 to that law, the fraction of the total energy which can be converted 

 into work depends upon the relative temperatures of the boiler and 

 condenser ; and it is, therefore, manifest that, as the temperature of 

 the boiler can not be raised indefinitely, it is impossible to utilize all 

 the energy which, according to the first law of thermo-dynamics, is 

 resident in the coal. 



On a sounder view of the matter, the efficiency of the steam-engine 

 is found to be so high that there is no great margin remaining for 

 improvement. The higher initial temperature possible in the gas-en- 

 gine opens out much wider possibilities, and many good judges look 

 forward to a time when the steam-engine will have to give way to its 

 younger rival. 



To return to the theoretical question, we may say with Sir W. 

 Thomson that, though energy can not be destroyed, it ever tends to 

 be dissipated, or to pass from more available to less available forms. 

 No one who has grasped this principle can fail to recognize its im- 

 mense importance in the system of the universe. Every change — 

 chemical, thennal, or mechanical — which takes place, or can take place, 

 in Nature does so at the cost of a certain amount of available energy. 

 If, therefore, we wish to inquire whether or not a proposed transfor- 

 mation can take place, the question to be considered is whether its 

 occurrence would involve dissipation of energy. If not, the trans- 



