On the Deduction of Thev.no fynamical Relations. 693 



temperature so low that the effect can be shown with a tube 

 of soft glass heated with a small Bunsen flame. 



There is an immense amount of work to be done with 

 iodine; and if we are ever to learn anything about these 

 enormously complicated absorption spectra, it is my belief 

 that the subject must be attacked along -the lines indicated 

 in this paper. This recent work puts a new light on some 

 of the results found with sodium vapour, which can now be 

 repeated to advantage. In every case it will be essential to 

 study the exciting line in connexion with the absorption 

 spectrum at the same point with a resolving power in the 

 neighbourhood of 300,000. In continuing the work [ propose 

 to have the exciting line under observation with an echelon 

 or large grating throughout the exposure. An attempt will 

 also be made to operate the mercury arc in a magnetic field. 



I have been aided in this investigation by a grant from 

 the Rumford Fund of the American Academy of Arts and 



Sciences. 



LXVTL On the Deduction of Thermodf/namical Relations. 

 By Dr. R. A. Houstoun, University of Glasgow*. 



IN a recent papert with the above title Prof. J. P. Kuenen 

 criticises the proofs of certain theorems of mine J, which 

 appeared in this journal. He finds that my results are valid 

 but arrives at them by another method. 



His criticism of the proof of my relation between torsion 

 and tension is owing to a misunderstanding due to the con- 

 densed \say in which the proof was given. I was considering 

 adiabatic changes although I was aware the relation he'd also 

 for isothermal changes. 



In the other point raised, i. e, the proof on p. 82 of the 

 paper on magnetization, I am at fault in not noticing that 

 that proof holds for an adiabatic process while the earlier 

 one is for an isothermal process, 



With reference to the main purpose of Prof. Kuenen's 

 paper, I doubt the superiority of his methods, I admit that 

 they are shorter and more elegant, but it is not at all so easy 

 to get a grasp of the physical ideas behind them. And that, 

 I think, is the real reason they have not proved popular.. 

 For, of course, they are not new. 



* Communicated bv the Author. 



t Phil. Mag. [6]xxiii. p. 752 (1912). 



X Phil. Mag. [6] xxi. p. 78 (1911) ; xxii. p. 740 (1911). 



Phil Mag. S. 6. Vol. 24. No. 142. Oct. 19.12. 2 Z 



