174 NOTICES OF THE MEETINGS [April 23, 
from different sources, which Melloni has also established, are nothing 
else than absorption of pEcuULIAR rays by each medium, not more 
anomalous 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 shewed that rays from the 
lamp transmitted in different proportions by various screens and 
then equalized, were afterwards transmitted by alum in equally va- 
rious 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 shews 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 (7. e. those at 
or beyond the red end) intercepted, as they are known to be by 
water: and this caused the position of the re/ative 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 ail 
hands admitted, viz. that the illuminating and heating powers follow 
very different laws, with relation to the different rays. 
