156 HISTORY OF THERMOTICS 



z, nearly impenetrable by light, allows a large portion of the heat 

 to pass. By passing the rays through various media, the heat may 

 Oe, as it were, sifted from the light which accompanies it. 



[2nd Ed.] [The diathermancy of bodies is distinct from their dia- 

 phaneity, in so far that the same bodies do not exercise the same powers 

 of selection and suppression of certain rays on heat and on light ; but 

 it appears to be proved by the investigations of modern thermotical 

 philosophers (MM. De la Roche, Powell, Melloni, and Forbes), that 

 there is a close analogy between the absorption of certain colors by 

 transparent bodies, and the absorption of certain kinds of heat by dia- 

 thermanous bodies. Dark sources of heat emit rays which are analo- 

 gous to blue and violet rays of light ; and highly luminous sources 

 emit rays which arc analogous to red rays. And by measuring the 

 angle of total reflection for heat of different kinds, it has been shown 

 that the former kind of calorific rays are really less refrangible than 

 the latter. 30 



M. Melloni has assumed this analogy as so completely established, 

 that he has proposed for this part of therrnotics the name Tkcrmo- 

 chrooloyy (Qu. Chromothermotics ?} ; and along with this term, many 

 others derived from the Greek, and founded on the same analogy. If 

 it should appear, in the work which he proposes to publish on this 

 subject, that the doctrines which he has to state cannot easily be made 

 intelligible without the use of the terms he suggests, his nomenclature 

 will obtain currency : but so large a mass of etymological innovations 



*/ ' v O 



is in general to be avoided in scientific works. 



M. Melloni's discovery of the extraordinary power of rock-salt to 

 transmit heat, and Professor Forbes's discovery of the extraordinary 

 power of mica to polarize and depolarize heat, have supplied thermo- 

 tical inquirers with two new and most valuable instruments. 31 ] 



Moreover, besides the laws of conduction and radiation, many other 

 laws of the phenomena of heat have been discovered by philosophers ; 

 and these must be taken into account in judging any theory of heat. 

 To these other laws we must now turn our attention. 



30 See Prof. Forbes's Third Serifs of Researches on Heat, Edlnb. R.S. Trans. 

 vol. xiv. 



31 For an account cf many thermotical researches, which I have been obliged 

 to pass unnoticed here, see two Reports by Prof. Powell on the present state 

 of our knowledge respecting Radiant Heat, in the Reports of the British Asso* 

 ciation for 1832 and 1840. 



