SECT. n. CAUSE OF TRANSPARENCY AND OPACITY. 37 



All ordinary transparent and colourless substances owe 

 their transparency to the discord which exists between 

 the oscillating periods of their molecules and those of 

 the waves of the whole visible spectrum. The general 

 discord of the vibrating periods of the molecules of 

 compound bodies with the light-giving waves of the 

 spectrum may be inferred from the prevalence of the 

 property of transparency in compounds, while their 

 greater harmony with the extra red periods is to be in- 

 ferred from their opacity to the extra red rays. Water 

 illustrates this transparency and opacity in the most 

 striking manner. It is highly transparent to the lumi- 

 nous rays, which demonstrates the incapacity of its mole- 

 cules to oscillate in periods which excite vision. It is 

 as highly opaque to the extra red oscillations, which 

 proves the synchronism of its periods with more of the 

 longer waves. If, then, to the radiation from any source 

 water shows itself to be eminently or perfectly opaque, 

 it is a proof that the molecules whence the radiation 

 emanates must oscillate in extra red periods. 



It has been already mentioned that many substances 

 which transmit radiant heat freely radiate badly, and 

 vice versa. Rock-salt is extremely permeable to radiant 

 heat but radiates feebly ; the reason according to Mr. 

 Tyndall is, that the motion of the molecules of the salt, 

 instead of being expended on the ether between them 

 and then communicated to the ether external to the 

 mass, is transmitted freely from molecule to molecule. 



Alum is exactly the reverse. Mr. Balfour Stewart 

 proved that alum is an excellent radiator, and Mr. 

 Tyndall proved it to be a very bad conductor, imparting 

 freely and with ease the motion of its molecules to the 

 external ether, and f for that very reason it finds difficulty 

 in transferring the motion from molecule to molecule. 

 The molecules are so constituted that when one of them 

 approaches its neighbour, a swell is produced in the 



