OF RADIANT HEAT THROUGH DIFFERENT BODIES. 63 
screens of differently coloured glass, in order to make them pass through 
one common screen of alum. - 
Screens from which the 100 rays issue 
that are made to fall successively 
on the same slice of alum. 
AGcARSE wy nites ed, 8. aa euioeetee ee eee Od 
STP POG castro cae. soso sissieaw garamacacut tes heal ebonlt 
See OFAN PS, on. ccs apeiashiags dead meerresiage aro 
Number of rays 
transmitted by this slice. 
Sek SN OUOW, cc ccnine ,aonibeetsnsacecs sa tomee fed. 
sey Uigpea CE 10101) ee Re Ree 
==). green (mineral) 04.02.45. .25.04<echeae-) i 
BRIS IG 6 a on cpt tesla anny abaepteoat head 
BD es AIMEE as teas lcekn « aaiecnitatunhta wend endl 
SNOT cea dnninanicas aaptgetens aap hae 
SOAS (OIACK) ons ccentoparsxasad pts 
We see here that the rays emerging from the red, orange, yellow, 
blue, indigo, and violet are transmitted through the plate of alum in the 
same proportion as the rays that issue from the white glass. The co- 
louring matter introduced into the composition of these different kinds 
of glass has no other effect than to extinguish part of the calorific sheaf 
which passed through the white, without perceptibly altering the rela- 
tions of quantity between the several species of rays of which that sheaf 
is composed: they act in respect to radiant heat just as brown or black- 
ish substances dipped in a transparent fluid would act in respect to light. 
But the case is different with respect to green and opake black; for 
these being introduced into the composition of glass, it will stop nearly 
all the rays that the alum is capable of transmitting. This effect arises 
from the green or opake colouring matter producing a certain modifi- 
cation in the diathermancy of the glass, and we have just seen that this 
species of calorific colouration is invisible and totally independent of co- 
loration properly so called, since it exists in bodies possessing the great- 
est transparency. It is then extremely probable that the black or the 
green should not be supposed to enter as mere neutrals into this phe- 
nomenon, which will thenceforth depend on such or such a property 
of these colouring materials. I have found, in fact, some green glasses, 
which produced a much feebler action than others of the same tint but 
possessing a less brilliant colouration. The green glasses which act most 
powerfully are of a bluish cast ; from which circumstance it would seem 
to follow that they contain a considerable quantity of oxide of copper. 
Whatever may be said of this singular property of green and black 
opake glasses and the cause by which it is produced, it is nevertheless 
an indisputable fact which every man can easily verify and of which we 
intend to give some new proofs presently. But it will perhaps be ad- 
visable previously to adduce the results furnished by several diather- 
