594 



NOTES TO BOOK X. 



(PA.) p. 549. THE diathermancy of bodies is, as 

 stated in the text, distinct from their diaphaneity, 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 colours by transparent 

 bodies and of certain kinds of heat by diathermanous bodies. 

 Dark sources of heat emit rays which are analogous to 

 blue and violet rays of light, and highly luminous sources 

 emit rays which are analogous to red rays. And by mea- 

 suring 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. See Prof. 

 Forbes's Third Series of Researches on Heat, Edinb. R. S. 

 Trans. Vol. xiv. 



M. Melloni has assumed this analogy as so completely 

 established, that he has proposed for this part of thermo- 

 tics the name Thermochroology (Qu. Chrothermotics ?); and 

 along with this term, many others derived from the Greek, 

 and founded on the same analogy. Tf 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 which he 

 suggests, his nomenclature will obtain currency ; but so 



