462 THE POPULAR SCIENCE MONTHLY. 



I repeat the experiment three or four times in succession, with the 

 same unfailing result. Here, as in the case of the sodium, the mag- 

 nesium surrounded itself for a moment by a cool envelope of its own 

 vapor, 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 flame, which you know to be 

 that of hydrogen, the product of combustion being water-vapor. Here 

 is another flame of a rich blue color, which the chemists present know 

 to be the flame of carbonic oxide, the product of combustion being 

 carbonic acid. Let the hydrogen-flame radiate through a column of 

 ordinary carbonic acid the gas proves highly transparent to the radi- 

 ation. Send the rays from the carbonic-oxide flame through the same 

 column of carbonic acid the gas proves powerfully opaque. Why 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 carbonic-acid gas, exactly as the rays from the intensely heated 

 sodium-vapor were quenched a moment ago by the cooler envelope 

 which surrounded it. Bear in mind the case is always one of syn- 

 chronism. It is because the atoms of the cold acid vibrate with the 

 same frequency 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, which, 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 impos- 

 sible to see the more secret and nobler works of Nature within the cor- 

 puscles, 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. Attempts have been made to push the analogy 

 still further by the discovery of a compound 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 can not reveal 

 an analogy between carbonic acid and bisulphide of carbon more occult 





