472 Intelligence and Miscellaneous Articles. 



nary amorphous charcoal is in general as bad a conductor as 

 paraffin, but at a red heat is a very good conductor. — Wiedemann's 

 Annalen, No. 7, 1884, p. 430. 



ON MR. MOON'S METHOD OF CALCULATING THE RATIO OF THE 

 TWO SPECIFIC HEATS OF GASES. 



To the Editors of the Philosophical Magazine and Journal. 



GrENTLEMEST, 



The paper by Mr. Moon in the last number of the Phil. Mag. 

 (p. 372) is an instructive example of the evil of an indistinct and 

 foggy style of writing. Had he set himself to think what was 

 the real gist of his argument, he could not have failed to per- 

 ceive that his method of calculating y is an old and well-known 

 one, taught to all junior students of thermodynamics, and first 

 virtually suggested by Mayer. 



The argument, put concisely but quite fully, is as follows : — 

 More energy is needed to warm gas at constant pressure than at 

 constant volume, because of the work done in expanding. Equating 

 the extra energy expended per gramme of gas to the external 

 work done (as Joule and Thomson have entitled us to do), we get 



j( c '_ c )dT=pdv; 



but the fundamental property of a gas, ^y = ET, combined with 

 the fact that the above change of volume occurred under constant 

 pressure, gives 



pdv = R,dT, 



whence , It 



0- C =j. 



Putting into this equation the numerical values selected by 



Mr. Moon for d, J, and E, - will come out equal to 1-41095, as he 

 correctly reckons. c 



Any one referring to Mr. Moon's paper will perceive how 

 elaborately he has involved this simple matter until it has become 

 laborious to disentangle his meaning. 



The difficulty of reading and appreciating what is good and new 

 in work would be greatly diminished if authors would more fre- 

 quently take pains to express themselves as clearly and distinctly 

 as possible. Your obedient servant, 



Univ. Coll., Liverpool, Oliver J. Lodge. 



.Oct. 13, 1884. 



