168 
FIFTH REPORT- 1835. 
at all so high as Melloni states it to be: thus, taking the effect 
without a screen as = 9-|-°, I found it 
with rock salt = 6^°, 
with rock crystal = \\°, 
and with alum . . =1°. 
Having removed the warm canister entirely out of the axis of 
the pile, so that the needle stood at zero, I then successively put 
the rock salt, the rock crystal, and the alum to the opening in 
the screen, and in one set of experiments 1 found the following 
results : 
with rock salt, needle marked about lj°, 
with rock crystal, —— above h°, 
with alum,- less than ^°, 
In another trial, with warmer water, the results were 
with rock salt . . . 2^°, 
with rock crystal above 1°, 
with alum . . nearly 1°. 
I may add that the effect on the needle appeared instanta¬ 
neous. These experiments are, confessedly, imperfect, and I 
only mention them for the purpose of pointing out a simple 
mode of answering the third and fourth questions on radiant heat 
contained in the second volume of the Reports of the Association, 
as to “ whether heat is transmitted through certain substances 
like light, or whether it is merely rapidly communicated by 
conduction , &c.*” If (as there may perhaps be some reason to 
suspect, even from these imperfect trials,) there be no direct 
transmission of simple heat, we may expect to find the same ef¬ 
fects produced by a given source of heat whether it be in or out 
of the axis of the thermoscope, provided the crystal and the 
canister be so arranged that they are at the same distance from 
each other and the inclination of their surfaces alike in each case. 
I may mention also, as bearing on this point, that with the canis¬ 
ter in the axis (its position, surface, and temperature being the 
same,) the effects on the crystals increased with the extent of 
the radiating surface, evidently from the crystals being more 
warmed , but that there was, nevertheless, no apparent change 
produced in the ratios of the effects with the different sorts of 
crystals* I may also refer to Melloni’s own remark with regard 
to the effect of increased thickness in the screen, 66 that the ob¬ 
struction is not at the first surface (as with light), but (as if the 
heat were conducted through the screen) in the substance of the 
screen itself.” 
* On this subject see Loud, and Edinb. Phil. Mar/-, vol. viii. p. 109. 
