Prof. Powell’s Papers on the Transmission of Calorific Rays. 111 
angled isosceles triangle, whose equal sides measure four inches, 
so that the canister being placed alternately in the different 
positions at R and S, it is evident that the lines drawn from 
any point of the radiating surface to a given point of the cry- 
stal are of equal length, and form the same angle with the cry- 
stal and canister in each case; while the other sides of the ca- 
nister cannot interfere with the effect. Under these circum- 
stances it would appear that any effect produced on the thermo- 
multiplier by the mere heating of the crystal should be pre- 
cisely; the same in both cases ; and any eacess of effect in the 
position R may be taken as the measure of the crystal’s dia- 
thermancy for the kind of heat which the canister radiates. 
The circle (in which the crystal is fixed) being moveable, the 
experiment can be repeated after turning the crystal round 
through 180° so as to verify the’ result. 
Having made use of the apparatus above described, in ad- 
dition to its furnishing abundant confirmation of M. Melloni’s 
statements, I think I have obtained a proof that rock crystal 
(about 3th of an inch thick) and other bodies, which are 
usually considered wholly impervious to the heat radiated from 
bodies at low temperatures, do transmit heat from a canister 
containing hot water, although the effect is obscured by the 
rapidity with which the crystal absorbs heat. I first put the 
canister as at S, and the needle almost (if not altogether) zn- 
stantaneously begins to move with a slow but rather steady 
motion, and at length stops (say) at 5°. The canister being 
removed the needle soon settles at zero again. ‘The canister 
is then placed at R, and immediately the needle begins to 
move (with more energy, however, than in the former case), 
and goes to 51°, from which it again returns to about 3°, and 
ultimately settles at 5°, as when the canister was at S. 
Thus, though there is no perceptible difference in the stati- 
cal effect, there appears to be a force acting on the needle at 
R (producing a larger are of vibration and in a shorter time) 
which, I believe, can only be attributed to rays of heat trans- 
mitted through the crystal. 
With regard to Professor Powell’s remarks on M. Melloni’s 
paper, in the Lond. and Edinb. Phil. Mag. for December, 
it appaers to me that the learned Professor has in some de- 
gree misapprehended M. Melloni’s observations. Professor 
Powell’s ingenious experiment went to prove that rays of heat 
issuing from aluminous heated body were transmitted freely, 
while those from a non-luminous source were apparently not so 
transmitted. 
The general fact on which Professor Powell founded the 
distinction M.Melloni admits, but he maintains (and on grounds 
