CONTRIBUTIONS TO MOLECULAR PHYSICS. 447 



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 re- 

 ferred to at the commencement of this lecture with those 

 of the ultra-red undulations. Hence, by gradually heat- 

 ing a platinum wire from darkness up to whiteness, we 

 ought gradually to augment the discord between it and 

 these vapours, and thus augment the transmission. Ex- 

 periment entirely confirms this conclusion. Formic 

 aether, 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 transparency also occur. For barely visible red- 

 ness formic aether 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 aether is more opaque than formic. This 

 result gives us a clear view of the relationship of the 

 two substances to the luminiferous aether. As we intro- 

 duce waves of shorter period the sulphuric aether 

 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 aether oscillate, on the whole, more slowly than 

 those of sulphuric aether. 



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

 with lampblack and filled with boiling water, the 

 opacity of formic aether 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 being 10 per cent., that of iodide of methyl is 



