1883.] on Thoughts on Radiation, Theoretical and Practical. 259 



vapour, which cut off the radiation from within, and thus produced 

 the darkness. 



And now let us pass on to an apparently different, but to a really 

 similar result. Here is a feebly luminous jflame, which you know to 

 be that of hydrogen, the product of combustion being water vapour. 

 Here is another flame of a rich blue colour, which the chemists 

 present know to be the flame of carbonic oxide, the product of com- 

 bustion being carbonic acid. Let the hydrogen flame radiate through 

 a column of ordinary carbonic acid — the gas proves highly transparent 

 to the radiation. Send the rays from the carbonic oxide flame through 

 the same column of carbonic acid — the gas proves powerfully opaque. 

 AVhy is this ? Simply because the radiant, in the case of the carbonic 

 oxide flame, is hot carbonic acid, the rays from which are quenched 

 by the cold acid exactly as the rays from the intensely heated sodium 

 vapour were quenched a moment ago by the cooler envelope which 

 surrounded it. Bear in mind the case is always one of synchronism. 

 It is because the atoms of the cold acid vibrate with the same fre- 

 quency as the atoms of the hot, that the pulses sent forth from the 

 latter are absorbed. 



Newton, though probably not with our present precision, had formed 

 a conception similar to that of molecules and their constituent atoms. 

 The former he called corpuscles, wliich, as Sir John Herschel says, 

 he regarded " as divisible groups of atoms of yet more delicate kind." 

 The molecules he thought might be seen if microscopes could be 

 caused to magnify three or four thousand times. But with regard to 

 the atoms, he made the remark already alluded to : — " It seems im- 

 possible to see the more secret and nobler works of nature within the 

 corpuscles, by reason of their transparency." 



I have now to ask your attention to an illustration intended to 

 show how radiant heat may be made to play to the mind's eye the part 

 of the microscope, in revealing to us something of the more secret and 

 noble works of atomic nature. Chemists are ever on the alert to 

 notice analogies and resemblances in the atomic structures of different 

 bodies. They long ago pointed out that a resemblance exists between 

 that evil-smelling liquid, bisulphide of carbon, and carbonic acid. In 

 the latter substance, we have one atom of carbon united to two of 

 oxygen, while in the former we have one atom of carbon united to two 

 of sulphur. Attemj)ts have been made to push the analogy still 

 further by the discovery of a comjiound of carbon and sulphur which 

 should be analogous to carbonic oxide, where the proportions, instead 

 of one to two, are one to one, but hitherto, I believe, without success. 

 Let us now see whether a little physical light cannot reveal an 

 analogy between carbonic acid and bisulphide of carbon more occult 

 than any hitherto pointed out. For all ordinary sources of radiant 

 i beat the bisulphide, both in the liquid and vaporous form, is the most 

 transparent, or diathermanous, of bodies. It transmits, for example, 

 30 per cent, of the radiation from our hydrogen flame, 10 per cent. 



