62 M. MELLOXI ON THE I.\rMEDIATE TRANSMIS.SION 



diathermancy of these bodies tlierefore aijproximates nearly to that of 

 the alum. The glass, the rock crystal, and the Iceland spar have evi- 

 dently a different diathermancy, for the rays which pass through them 

 are less transmissible by tlie invariable screen. The same may be said 

 of borax, adularia, and carbonate of ammonia. As to the heat emerging 

 from rock salt (limpid or dull) it acts in a manner similar to that in which 

 the unobstructed light of the lamp woidd. The reason is evident, since 

 the salt, acting equally on the different species of calorific rays, must 

 transmit them all without reflecting their relative properties in any man- 

 ner whatsoever. 



These facts then completely confirm the conclusions which we had 

 drawn from the preceding experiments : namely, that, 1st, flame sends 

 forth rays of several kinds ; 2nd, that diaphanous colourless bodies, with 

 the exception of rock salt, act so as to extinguish certain caloric rays 

 and allow others to pass, just as coloured media act in respect to light. 



Here a very interesting question is naturally suggested. If the dia- 

 thermancy or quality which constitutes the tint of a medium relatively 

 to the radiant caloric is invisible, what part then do colours act in the 

 transmission of heat ? 



When the quantity of radiant heat that passes through coloured glass 

 is measured, it is always found to be less than that which passes through 

 white glass of the same thickness. The difference indeed is sometimes 

 considerable, though having no apparent relation to the prismatic order 

 or intensity of the colour. We have already remarked this in the first 

 memoir, and the truth of the remark will be readily admitted by any 

 one who casts an eye over the following little table. 



Screens of glass exposed to the m • • 



J. .. ''.. T ,. n- 1 Iransmissions 



radiation 01 a Locatelli lamp. ^ />■■«« ^-i 



,<-, .1 . , imm o- N out oi 100 ravs ot lieat. 



(Common tliickness l'"'"-8o.) •' 



Glass, white 40 



— red (deep) 33 



— orange 29 



— yellow (brilliant) 22 



— green (apple) 25 



— green (mineral) 23 



— blue 21 



— indigo 12 



— violet (deep) 34 



— black (opake) 17 



It is therefore not to be doubted that an absorption of caloric is caused 

 by the colouring matter. But is the power of absorption elective like the 

 action of the invisible calorific tints in colourless diaphanous bodies, or 

 does it affect all sorts of rays indiscriminately? We are about to inves- 

 tigate this point by means of experiments similar to the preceding, in 

 which we have taken equal quantities of heat issuing from different 



