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 the results, in hundredth parts, of the constant 
quantity of heat that falls on the plate of alum: 
Screens from which there issue 100 rays 
of heat which are made to fall succes- Number of rays transmitted 
sively on the same plate of alum. LG aaah 
VGISETeSH A Ce. be: ftiatdilne cates 9 
Rock salt (limpid).............0.00000 9 
Baek.salt (ull picasiisasaSsisoniies -nryiey 9 
IBOLALe, OF SOMA,» . cicssiccinccaiejosiqe's.ciea.ecs 11 
ACULATIA TEISPAD aon con onsnncnesonsees 14 
Weeland SPA ercieecsseceendadeerces’ 2 
HOGER CHYSEAN Sasefecsc sce sac enwens stews? aD 
Miirrorelass Oi sak Ga eens. 27 
Carbonate of ammonia .............65 31 
Sulphateof limes Meo. siccssidadecces ons 72 
Tartrate of potash and soda. ......... 80 
ir Cha Ide rete ace Soe eaters a 85 
PAIN ee sR aoe an ees ose tcaetesue A COU 
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. Ampére, who has conti- 
nued to assist me with his valuable advice in the composition of this Memoir, 
for which I here take the opportunity to tender him my grateful acknowledge- 
ments. 
