OS M- MELLON! ON THE IMMEDIATE TRANSMISSION 



Several of the numerical results contained in this table may be verified 

 by calculation. 



For, when two plates of different kinds are exposed together to the 

 radiation of the source, their position relative to the entrance and the 

 issue of the calorific rays does not affect the quantity of heat which passes 

 through this system. This is easily proved by putting the first plate in 

 the place of the second ; for the thermomultiplier, notwithstanding this 

 change of order, continues to mark the same degree of its scale. Let 

 us now take two plates and place them alternately in each of the two 

 positions, for instance, the plate of alum and the chromate of potash. 

 These two substances, exposed separately to 100 rays of heat emanating 

 directly from the source, transmit 9 and 34 respectively. The quantities 

 of heat that should fall on each of the two plates in order that 100 may 

 emerge in each case is easily determined by these simple proportions; 



9 : 100 : : 100 : x, 

 34 : 100 : : 100 : x, 



which give 1111 for the alum and 294 for the chromate of potash. Now 

 we know by experiment that chromate of potash exposed to 100 rays is- 

 suing from alum transmits 57, and that alum exposed to 100 rays issuing 

 from chromate of potash transmits 15. 



But the order of succession has no influence on the transmission of 

 the pair : let us therefore reverse the system only in one case or the 

 other. We shall then have the same plates exposed in the same man- 

 ner to the two radiations of 1 1 1 1 and 294. The quantities transmitted 

 under both circumstances should accordingly be proportional to the 

 incident quantities, as is actually proved within the limits of approxi- 

 mation compatible with the nature of the experiments; for we have, 



57 ; 15 : ; 1111 : 294. 

 The table contains ten pairs which are submitted in both ways to the 

 radiations of the source ; there are in it consequently twenty numbers 

 which should be in proportions analogous to the preceding. It is evi- 

 dent too that these calculations require that the five plates emitting the 

 100 rays which fall successively on the whole seiies of diathermanous 

 bodies should be those that are indicated by the same names in the first 

 column. I have accordingly taken care that this condition should be 

 satisfied. 



The bodies submitted to the heat emerging from the screens present 

 no longer the same order of transmission that they presented under the 

 immediate action of the radiation of the lamp. The changes which 

 take place have no apparent regularity whether we compare one scries 

 with another or consider only the different terms of the same series. 

 Thus glass, Iceland spar, and rock crystal are more diathermanous to 

 the heat emerging from the five screens than to that which comes di- 



