CONTRIBUTIONS TO MOLECULAR PHYSICS. 395 



ture meanwhile varying between the widest possible 

 limits. Their comparative opacity to the ultra-red 

 rays shows the general accord of the oscillating periods 

 of the vapours referred to at the commencement of this 

 lecture with those of the ultra-red undulations. Hence, 

 by gradually heating a platinum wire from darkness 

 up to whiteness, we ought gradually to augment the 

 discord between it and these vapours, and thus aug- 

 ment the transmission. Experiment entirely confirms 

 this conclusion. Formic ether, for example, absorbs 

 45 per cent, of the radiation from a platinum spiral 

 heated to barely visible redness; 32 per cent, of the 

 radiation from the same spiral at a red heat; 26 per 

 cent, of the radiation from a white-hot spiral, and only 

 21 per cent, when the spiral is brought near its point 

 of fusion. Remarkable cases of inversion as to trans- 

 parency also occur. For barely visible redness formic 

 ether is more opaque than sulphuric; for a bright red 

 heat both are equally transparent; while, for a white 

 heat, and still more for a higher temperature, sulphuric 

 ether is more opaque than formic. This result gives 

 us a clear view of the relationship of the two substances 

 to the luminiferous ether. As we introduce waves of 

 shorter period the sulphuric ether augments most 

 rapidly in opacity; that is to say, its accord with the 

 shorter waves is greater than that of the formic. Hence 

 we may infer that the atoms of formic ether oscillate, 

 on the whole, more slowly than those of sulphuric 

 ether. 



When the source of heat is a Leslie's cube coated 

 with lampblack and filled with boiling water, the opac- 

 ity of formic ether in comparison with sulphuric is 

 very decided. With this source also the positions of 

 chloroform and iodide of methyl are inverted. For 

 a white-hot spiral, the absorption of chloroform vapour 



