Royal Institution. 537 



is applied indiscriminately to a heterogeneous body of rays ; out of 

 which some species of rays are entirely stopped, others entirely trans- 

 mitted ; and the great differences in " diathermaneity " for heal from 

 different sources, which Melloni has also established, are nothing else 

 than absorption of peculiar rays by each medium, not more anoma- 

 lous than the corresponding absorptions of luminous rays by different 

 transparent media so little as yet reduced to law. 



While rock-salt is analogous to colourless media for light, alum on 

 the other hand is totally impermeable by heat from dark sources, and 

 partially so by rays from the lamp ; that is, wholly impermeable for 

 that portion of the rays which are of the same kind as those from non- 

 luminous sources, and permeable to the others. 



By other sets of experiments Melloni showed that rays from the 

 lamp transmitted in different proportions by various screens and then 

 equalized, were afterwards transmitted by alum in equally various 

 proportions; or as he expresses it, " possess the diathermancy peculiar 

 to the substances through which they had passed." 



But this implies no new property communicated to the rays. 

 It shows that as different specific rays out of the compound beam 

 were transmitted in each case by the first screen, alum, though im- 

 pervious to the lower heating rays, is permeable by these higher 

 rays ; and in different degrees according to their nature ; an effect 

 simply dependent on the heterogeneity of the compound rays from 

 a lamp. 



Again, with differently coloured glasses peculiar differences of dia- 

 thermaneity were exhibited with rays from a lamp, incandescent 

 metal, and the sun ; but not more various or anomalous than the 

 absorption of specific rays of light. 



And besides considerations of this kind, it must always be borne 

 in mind that a blackened surface (like that which was used in all 

 these experiments) itself is unequally absorptive for the different rays. 



The solar heat being freely transmissible through all colourless 

 transparent media along with the light, there would be no peculiar 

 advantage in experimenting on the solar spectrum formed by a 

 rock-salt prism. Melloni however with such a prism, on interposing 

 a thick screen of water, found the most heating rays (2. e. those at 

 or beyond the red end) intercepted, as they are known to be by 

 water ; and this caused the position of the relative maximum to be 

 apparently shifted higher up in the spectrum, even to the position of 

 the green ray. 



On the other hand, many coloured glasses, he found, absorbed the 

 rays in various proportions, yet they left the point of maximum heat 

 unaltered; i.e. though variously absorptive for the higher rays, they 

 were not of a nature to stop the lower, or most heating rays. 



One result indeed is recorded which seems at variance with all 

 other experiments on the solar rays : a peculiar green glass (tinged 

 by oxide of copper) was found to absorb so entirely all the most 

 heating rays that the remaining portion produced no heat, though 

 when concentrated by a lens they gave a brilliant focus. Speaking 

 generally, however, these experiments only confirm what is on all 



