458 Prof. Donkin on certain statements in Elementary Works 



limits of the visible spectrum. The substance which is thus im- 

 pervious to the luminous rays is moreover the very one from 

 which the whitest light of our lamps is derived. It can absorb 

 all the rays of the visible spectrum, it can also emit them. But 

 though in a far less degree than iodine, lampblack is also to 

 some extent transparent to the longer undulations. Melloni 

 was the first to prove this ; and in an experiment described in a 

 former memoir, I myself found that 30 per cent, of the radiation 

 from an obscure source found its way through a layer of lamp- 

 black which cut off totally the light of the most' brilliant jet of 

 gas. I shall have occasion to show that, for certain sources of 

 heat of long period, between 40 and 50 per cent, of the entire 

 radiation is transmitted by a layer of lampblack which is per- 

 fectly opake to our most brilliant artificial lights. Hence, in the 

 case of lampblack, while accord exists between the periods of its 

 atoms and those of the light -exciting waves, discord, to a consi- 

 derable extent, exists between the periods of the same atoms and 

 those of the extra-red undulations. 



[To be continued.] 



LV. Note on certain statements in Elementary TVorks concerning 

 the Specific Heat of Gases. By Professor Donkin, F.R.S.* 



A YOUNG student of natural science showed me a few days 

 ago the following statement in Galloway's ( Second Step 

 in Chemistry ' (London, 1864). It had naturally surprised him, 

 and he asked for an explanation, which I was quite unable to 

 give. 



" From the calculations of Laplace and Poisson, and the expe- 

 riments of Clement and Desormes, of De la Roche and Berard, 

 and of Gay-Lussac and Dulong, it has hitherto been assumed 

 that the specific heat of a gas under a constant pressure is 

 always greater than 8 the specific heat under a constant volume ; 

 but M. Regnault has lately found, by an entirely new method, 

 that the difference between the two kinds of specific heat is either 

 null or extremely small." (P. 585, paragraph 1321.) 



This paragraph is not accompanied by any note or reference, 

 but it is enclosed in inverted commas, and I soon discovered that 

 it is a translation of a passage in Ganot's Traite elementaire de 

 Physique (Paris, 1859). See p. 312, end of paragraph 334. 



There is an English translation of Ganot, in which the same 

 passage occurs, and is left, as it is in the original, without note 

 or comment. 



I applied for further information to some of my scientific 



* Communicated bv the Author. 



