ON THE RADIATION OF HEAT. 
I GO 
With reference to the thermomultiplier, it will, I believe, be 
found to be most useful in experiments with screens of various 
sorts ; its delicacy is, no doubt, very great, but its expensive na¬ 
ture is unfavourable to its general introduction. I believe the 
eetherial thermometer will be found at least as delicate an indi¬ 
cator of small changes of temperature; but the instrument may 
be made still more sensitive by using aether saturated with am¬ 
monia, or incomparably more delicate still by substituting other 
liquids, as Faraday’s “ volatile carburetted hydrogen,” or the 
liquids of the condensed gases, for instance, sulphurous acid or 
(what I should prefer) euchlorine. It may be thought absurd 
to suggest such an application of these latter bodies, but I can 
vouch for its practicability, at least with reference to sulphurous 
acid, as I actually produced the effect by introducing quicksilver 
and sulphuric acid into one of the balls of a differential ther¬ 
mometer, and after it was sealed applying heat to the ball. Not 
being certain how far I had succeeded, I incautiously applied my 
hand to the other ball, on which the tube instantly broke where 
it had been joined, and the colourless liquid, which had pre¬ 
viously nearly half filled this ball, entirely disappeared, with an 
overpowering evolution of sulphurous acid gas. I have not had 
time since to make another trial, but I hope to be able to accom¬ 
plish it in the course of the next month. 
Should the experiments mentioned in the beginning of this 
paper be thought to throw any light on a difficult subject, I feel 
it but justice to state, in concluding, that any advantage which 
science may derive from their execution is essentially due to the 
, spirited individuals who originated the British Association, as 
though forming a portion only of a series planned many years 
ago, I should probably have never carried them into effect if it 
had not been for the stimulus produced by reading Professor 
Powell’s excellent Report in the first volume of the Transac¬ 
tions of the Association. 
