Mr. J. Gill on the Dynamical Theory of Heat. 109 



of its cooling being interfered with by the shelter of the test- 

 glass ; but on taking it out of the glass, and. placing it on the 

 window-frame, it began to act in a few hours, and has behaved 

 well for many weeks. 



King's College, London. 

 18th May, 1863. 



XIV. On the Dynamical Theory of Heat. 

 By Joseph Gtll, Esq. 



To John Tyndall, Esq., F.R.S. fyc. 

 Sir, 



AT the present time, when the subject of thermodynamics is 

 beginning to attract more general attention from the 

 scientific public, every little contribution of new light may be 

 useful in aiding the progress of discovery in " a region which," 

 as you have justly remarked, " promises possessions richer than 

 any hitherto granted to the intellect of man." As an obscure 

 labourer in this grand field of science, I ought perhaps to apo- 

 logize for asking from you, as an Editor of the Philosophical 

 Journal, the favour of making public, through its pages, a few 

 remarks which may not be useless towards clearing away some 

 of the obscurity which appears to me still to overhang the very 

 first steps of our progress ; and in explanation I may be allowed 

 to say that the subject has been of deep and fascinating interest 

 to me, from the dim foreshadowing of the theory sketched by 

 Seguin twenty-four years ago, and the powerful outlines drawn 

 soon after by the master hand of Mayer, down through the mass 

 of important experimental evidence of Joule, backed by the labours 

 of Thomson, Clausius, Eegnault, and Helmholtz, to Professor 

 Rankine's thermo-dynamics in his recent work on Prime Movers. 

 And while I have been endeavouring to grasp in some degree 

 the philosophy of the subject from the works of others, I have 

 not omitted to aid my own speculations and researches with 

 numerous original experiments made under circumstances of no 

 little difficulty, but, I venture to hope, not utterly in vain. 



I ought candidly to declare at once that my long investigation 

 of the practical working of the steam-engine, and my numerous 

 experiments with hot air as a motive agent, forced me many years 

 ago reluctantly to doubt that any heat, contained under the com- 

 mon form of heat by the elastic fluids, is directly converted into 

 work in the action of thermic prime movers ; and it has been a 

 source of sincere regret to me that a beautiful theory, elaborated 

 with admirable skill by some of the first intellects of the age, and 

 which, applied to some grand cosmical phenomena, gives results 



