RADIANT HEAT, AND ITS CONVERSION THEREBY INTO SOUND. 
295 
A similar deportment may be detected in liquids and solids. The quantity of 
iodine vapour generated at ordinary temperatures is so small that its action on radiant 
heat is, as might be expected, insensible. But iodine itself, when liquefied by a 
powerful solvent, behaves as an almost perfectly transparent body to the obscure 
calorific rays, even when it is able to extinguish totally the light of the sun. Liquid 
bromine is also highly diathermanous. The same may be said of phosphorus. In 
Melloni’s table, Sicilian sulphur comes next to rocksalt in transmissive power. A 
concentrated solution of sulphur in bisulphide of carbon exerts no sensible action on 
radiant heat. By fusing together iodine and sulphur Professor Dewar has produced 
a “ray filter ” which separates with extreme sharpness the visible from the invisible 
rays. The remarkable diathermancy of certain specimens of vulcanite, brought to 
light in the experiments of Mr. Graham Bell and Mr. Preece, is probably due to 
the sulphur they contain. Melloni showed that lampblack is to some extent dia¬ 
thermanous. But when a suitable source of heat is chosen, lampblack proves far more 
pervious to radiant heat than Melloni found it to be. An opaque layer of this 
substance transmits 41 per cent, of the radiation from a hydrogen flame. Were the 
lampblack optically continuous, the transmission would, doubtless, be still greater. 
An opaque solution of iodine transmits 99 per cent, of the radiation from the same 
source, while a layer of pure water 0’07 of an inch in thickness, transmits only 2 per 
cent, of the radiation from a hydrogen flame. Such results indicate that a profound 
change in the relation of ponderable matter to the luminiferous ether accompanies the 
act of chemical combination. 
One of my principal aims in the Bakerian Lecture of 1861 was to illustrate the hold 
which experiment had obtained of a subject previously considered intractable. The 
densities of the gases and vapours employed were therefore varied within wide limits. 
Tn the experimental tube first made use of, a full atmosphere of olefiant gas absorbed 
more than 80 per cent, of the entire radiation, and it was therefore evident that a small 
fraction of an atmosphere of such a gas would exert a measurable action. On trial, it 
was found possible to measure the absorption of roulooth of an atmosphere of olefiant 
gas. The action of this gas was determined at sixteen different densities, the absorp¬ 
tion, as long as the density was very small, being accurately proportional to the 
quantity of gas present. Similar experiments were made, and similar results obtained 
with other gases. The action of sulphuric ether vapour upon radiant heat was proved 
to be still more powerful than that of olefiant gas. The vapour was first carried into 
the experimental tube by a current of dry air; and the pure vapour was afterwards 
examined at seventeen different densities. Bisulphide of carbon was tested at twenty 
different densities; amylene at ten; benzol at twenty; and so of the others. 
Considering the views previously entertained regarding the diathermancy of gases 
and vapours I was naturally impressed with these results. Sceptical when I first 
observed them, I scrutinised them closely, until repeated scrutiny abolished every 
