OF RADIANT HEAT THROUGH DIFFERENT BODIES. 61 



grade movement is sometimes so marked that the needle nearly resumes 

 its natural position of equilibrium. 



If instead of alum other substances were employed as the invariable 

 plate on which the rays issuing from each diaphanous body are succes- 

 sively made to fall, we should still observe differences in the correspond- 

 ing deviations of the galvanometer; but they would be in general of a 

 less decided kind. It is on this account that we have preferred the 

 alum. 



The following are tlie results, in hundredth parts, of the constant 

 quantity of heat that falls on the plate of alum : 



Screens from which there issue 100 rays -kt i /. ■ . ■, 



ofheat which are made to fall succes- Number of rays transmitted 



sively on the same plate of alum. ">" ^^'' P'^*^' 



Noscreen 9 



Rock salt (limpid) 9 



Rocksalt (dull) 9 



Borate of soda 11 



Adularia felspar 14 



Iceland spar 22 



Rock crystal 25 



Mirror glass 27 



Carbonate of ammonia 31 



Suljihate of lime 72 



Tartrate of potash and soda 80 



Citric acid 85 



Alum 90 



We see that radiations of the same intensity emanating from the dia- 

 phanous and colourless bodies contained in the tables pass through the 

 same plate of alum in very different quantities. In the same manner 

 sheaves of luminous rays issuing from different coloured media are 

 transmitted some in greater and others in less proportions by a second 

 transparent substance equally coloured, as the tint of each medium hap- 

 pens to be more or less analogous to that of the invariable substance 

 through which they are to pass. 



The calorific rays issuing from the diaphanous screens are therefore 

 of different qualities and possess (if we may use the term) the diather- 

 mancy * peculiar to each of the substances through which they have 

 passed. The citric acid, the tartrate of potash and soda, and the sul- 

 phate of lime transmit rays which pass abundantly through alum ; the 



• I employ the word diathermancy as the equivalent of calorific coloration or 

 calorific tint, lest the latter should be confounded with tints or colours properly 

 so called. The word has been suggested to me by M. Ampere, wlio has conti- 

 nued to assist me with his valuable advice in the composition of this Memoir, 

 for which I here take the opportiMity to tender him my grateful acknowledge- 

 ments. 



