From Joseph Gill

ON THE DYNAMICAL THEORY OF HEAT. | To John Tyndall, Esq., F.R.S., &c.

SIR,

I AGREE with the Editors of the Philosophical Magazine, that it is not desirable that my remarks on the dynamical theory¹ should give rise to prolonged discussion. What is wanted is experimental proof of the direct conversion of heat into mechanical work; and after what has been so ably achieved by Dr. Joule for physical science, in proving beyond a doubt the conversion of work into heat, and determining the numerical equivalent of the transformation, surely it is reasonable to expect that every effort should be made to obtain positive physical evidence of the reality of the conversion of heat into work. I thank the Editors for kindly allowing me to state the difficulties which I have encountered in this pursuit, and the consequent doubts which have presented themselves of the completeness of the dynamical theory as it now stands; and I beg to avail myself further of their courtesy to clear up in a few lines some want of perspicuity in my last letter, which may in some degree have influenced the remarks of Dr. Rankine and of Mr. Croll in the Philosophical Magazine for March.²

Dr. Rankine is right in pointing out my inadvertence in overlooking the application of the second law of the action of heat in two instances. In the first, namely, my statement that the regenerator might take up and restore all the heat remaining in air after it has done work by expansion, my omission was but half unconscious, and arose from my having uppermost in my mind the ideas of thermodynamic action published by me some years ago; and had I entered more into detail on this point in my last letter it would have been seen that my oversight was more in appearance than in reality, as the difficulty of the hypothetical case proposed by me would not have been removed by a formal application of the second law. In the second case, the estimate I gave of the useful effect of steam-engines was the old statement of M. Regnault; and I should have stated that it was quoted by me from M. Verdet’s Second Lecture on the Thermo-dynamical Theory of Heat (Paris, 15^th February 1862, section 2). Of course, in this case the application of the second law made it requisite to suppose (in addition to M. Regnault’s deductions from the mere calculation of the latent heat) the condensation of some steam as a result of the actual conversion of heat into work; but as this is the very point at issue, and a point on which the grand question of the practical improvement of our heat engines mainly depends, I may be allowed to state that, as far back as 1843, I published results of my experiments,³ showing that the whole heat resulting from the condensation of steam in the cylinder of an engine working expansively should be found in the condenser, after allowing for known losses from other causes—a result which subsequent experience has steadily confirmed.

I allow the propriety of Dr. Rankine’s disapprobation of my remark, that M. Hirn⁴ was borne down by scientific authority in altering his opinions on the results of his own experiments, as, after all, the publication of this change of opinion was his own free act. Though I said “apparently borne down,” the expression was inconsiderate, and I freely retract it. I cannot forget that I am indebted to the courteous kindness of Dr. Rankine for spontaneously sending me valuable scientific information, which in my comparatively isolated position here I could not otherwise have easily obtained; and though I reluctantly differ from him on some points of scientific theory, my high appreciation of his intellectual superiority will always cause me to respect and value his opinion, whether for censure or approbation.

While thanking Mr. Croll for his remarks on my uncertainty about the estimate of work done by heated air in the shape of raising the weight of the atmosphere, and on the specific heat of air, I would beg to refer back to my chief difficulty on this latter point, which perhaps I did not state with sufficient clearness, viz. the disappearance of energy in the supposed experiment. What I meant to express was this:—There must be more heat in the air after expansion than before, because the energy which might have been availed of by allowing the atmosphere to fall back into the vacuum, and thus produce work, has disappeared, and we can imagine no other equivalent for it. And the specific heat of the expanded air, or its capacity for heat, is greater because it does not show this increase of quantitative heat.

Not having been able to make the result of experiment agree with the dynamical theory as to the direct conversion of heat into work, I have been long pondering on some consistent explanation of the physical facts, which have continued to present themselves in my researches. On this subject I had a thing to say, but let it go for the present, and now I will return with renewed energy to the experimental investigation of this very interesting but still perplexing subject.

Believe me Sir, your most obedient Servant, | JOSEPH GILL. | Palermo, April 4, 1864.
J. Tyndall, ‘On the Dynamical Theory of Heat’, Phil. Mag., 27 (1864), pp. 477–8