194 Prof. Rankine on the Dynamical Theory of Heat. 



instance like a diamond, which depresses minute particles of 

 glass, and through the wedge- action of which a progressive 

 linear cracking is produced which renders fracture possible. A 

 mere scratch suffices neither for glass nor for ice. It is worthy 

 of notice that these clear sharp cracks are possible in ice which 

 is in contact with water in a warm room — a proof that ice 

 preserves its brittle character at temperatures lying close to its 

 melting-point. The breaking off at the crack is, on the other 

 hand, no longer possible on account of the regelation in the 

 upper part of the crack. 



I remain, 



Yours truly, 



E. Reusch, 

 Professor of Natural Philosophy, 

 Tubingen. 



XXXIV. On the Dynamical Theory of Heat. 

 By W. J. Macquorn Rankine, C.E., LL.D., F.R.SS.L. %E. 



To the Editois of the Philosophical Magazine and Journal, 

 Gentlemen, 



TO state arguments in reply to the objections raised by Mr. 

 Gill to the Dynamical Theory of Heat would be merely to 

 repeat what has been published in various forms during the last 

 fifteen years. It is possible, indeed, that new matter for discus- 

 sion might arise from his statement that there is probably some 

 "concealed source of error " in the experiments of M. Regnault 

 on the specific heat of air, but not until Mr. Gill shall have 

 specified the nature of such source of error and the probable 

 amount of its effects; and then the option of answering him 

 would rest with M. Regnault. My object, therefore, in writing 

 this paper is not to argue against any of the opinions expressed 

 by Mr. Gill, but to point out that the account which he gives of 

 the dynamical theory of heat is incomplete and erroneous, through 

 the omission to take any notice of one of its two fundamental 

 principles, — the consequence being that propositions to which he 

 objects on the supposition that they belong to that theory have 

 no real connexion with it, or are directly opposed to it. 



The fundamental principle which Mr. Gill has omitted to con- 

 sider is that which has been called the " Second Law M of the 

 mechanical action of heat. It is in some respects identical with 

 that which, under the name of " Carnot's Law," formed a part 

 of the Theory of Substantial Caloric ; and may be treated either 

 as an independent principle (as was first done by Professor 

 Clausius in 1849), or as a consequence of the hypothesis that 



