416 FRAGMENTS OF SCIENCE 



the cold gas belonging to the ultra-red portion of the 



spectrum. 



It will be seen from the foregoing remarks and experi- 

 ments how impossible it is to determine the effect of tem- 

 perature pure and simple on the transmission of radiant 

 heat if different sources of heat be employed. Through- 

 out such an examination the same oscillating atoms ought 

 to be retained. This is done by heating a platinum spiral 

 by an electric current, the temperature 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 vapors referred to at the com- 

 mencement of this lecture with those of the ultra-red un- 

 dulations. Hence, by gradually heating a platinum wire 

 from darkness up to whiteness, we ought gradually to 

 augment the discord between it and these vapors, and 

 thus augment the transmission. Experiment entirely con- 

 firms this conclusion. Formic ether, for example, absorbs 

 45 per cent of the radiation from a platinum spiral heated 

 to barely visible redness; 82 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. Ee- 

 markable cases of inversion as to transparency 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 relation- 

 ship 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 



