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



force, — a theory which appears to shed a fresh light upon almost 

 every branch of physical inquiry, and to impart anew interest to 

 subjects the study of which was already beginning to settle down 

 into a quiet current of fixed opinions. 



The grand simplicity of the idea of the conservation of force, 

 together with the tendency to generalize which influences most 

 inquirers in scientific matters, would naturally lead to the belief 

 that if mechanical energy is directly convertible into heat, heat 

 should be also directly convertible into mechanical energy, and, 

 of course, in the same proportion. Hence it is assumed that 

 when a hot perfect gas performs work by expansion in a cylinder 

 supposed to be neutral to the thermometric effects of heat, and 

 having a piston moving without friction, it must naturally lose 

 as much heat in raising a weight of 772 pounds one foot as 

 would raise the temperature of one pound of liquid water 1 de- 

 gree of Fahrenheit's scale. By the same reasoning, steam, in 

 performing work in vessels supposed to be equally neutral to 

 heat, should lose exactly the same quantity of heat in doing the 

 same quantity of work ; and as perfect steam (neither super- 

 heated nor containing water in suspension) cannot lose heat 

 without undergoing a corresponding amount of condensation, it 

 is assumed that in the working of the steam-engine as much 

 steam is condensed (theoretically) as corresponds to the amount 

 of work performed in the proportion of Joule's equivalent, and 

 consequently that the quantitative heat of the steam passing into 

 the condenser is by so much less than the total heat of the steam 

 furnished by the boiler to the engine. 



More than thirty years ago Seguin, in his interesting work 

 ' On the Influence of Railways/ endeavoured to trace the dyna- 

 mical relation which he saw must exist between the heat applied 

 to the boiler and the energy developed by the engine; and it is 

 remarkable that, in many experiments made by him on the actual 

 working of steam-engines, with the express purpose of proving 

 the disappearance of quantitative heat which he thought must be 

 converted into the work done, he could not detect the expected 

 disappearance of heat. 



Soon afterwards Mayer published some very interesting spe- 

 culations on the mechanical equivalent of heat ; and, from theo- 

 retical considerations on the effect of heat in increasing the elastic 

 pressure of air, he deduced a numerical value of the dynamical 

 equivalent, which agrees very closely with the result of Dr. Joule's 

 independent and highly philosophical experiments. These ex- 

 periments proved indisputably that mechanical energy is directly 

 convertible into heat ; but so far no experiments seem to have 

 proved with equal clearness that heat was directly convertible 

 into mechanical work. More recently this part of the inquiry 



