Mr. J. Gill on the Dynamical Theory of Heat. 85 



thematical knowledge requisite for treating these subjects on the 

 higher scientific plane to which their full theoretical development 

 more properly belongs. It was well remarked by Dr. Lardner, 

 that " the phenomena of heat all admit of being explained with- 

 out the aid of abstruse reasoning, technical language, or mathe- 

 matical symbols. The subject abounds in examples of the most 

 felicitous processes of induction, from which the general reader 

 may obtain a view of that beautiful logic, the light of which 

 Bacon first let in on the obscurity in which he found physics 

 involved." I may truly say that after the perusal of your book 

 I understand the subject better than I did before ; but still some 

 of my doubts are unresolved, as you will perceive from the fol- 

 lowing statements. 



Although a fixed equivalence undoubtedly exists between the 

 heat generated by friction and the energy expended in producing 

 the friction, experiment proves that elastic fluids suffer no dimi- 

 nution of heat in the act of producing work, but only a fall of 

 temperature. I have long perceived the difficulties of admitting 

 this hypothesis, as it involves the apparent paradox of a conti- 

 nual investment of energy in the production of molecular motion, 

 of the reconversion of which back again into energy no account 

 can be given. But the results of numerous and varied experi- 

 ments leave me no alternative, and I must repeat my conviction 

 that heat runs down but does not disappear in the production of 

 work. Even admitting that quantitative heat disappears in the 

 process, still many reasons might be given for doubting that 

 any fixed equivalence exists between the heat disappearing and 

 the work done ; but for the present I will confine myself to the 

 simple consideration that, with the disappearance of equal quan- 

 tities of heat in the expansion of air of constant initial density, 

 different quantities of work should be produced according to 

 the mode of conducting the process. 



Imagine a cylinder of 2 cubic feet capacity to contain 1 cubic 

 foot of air of atmospheric pressure at the temperature of 32° F. 

 confined by a piston resting at half stroke, supposed to be with- 

 out weight or friction. Let the piston be fixed in this position, 

 and heat applied to the air until its temperature is raised 490° 

 above the initial point, and let the quantity of heat required to 

 produce this effect be represented by 100. In this process the 

 air is simply heated, and although statical tension is developed, 

 no dynamical effect is produced, it is merely a preparation for 

 work — a transfer of heat from one substance to another if the 

 heat is taken either ready made, or produced chemically, or a 

 change of some form of energy into heat by friction ; but, as 

 long as no work is done by the least air, things are considered 

 to remain in a state of balance or equilibrium. 



