160 Prof. Tyndall — Contributions to Molecular Physics. [Mar. 17, 



carbonic acid; but for the radiation from the carbonic oxide flame at a 

 common tension of one inch of mercury, while carbonic acid absorbs 50 per 

 cent., olefiant gas absorbs only 24. Thus we establish the coincidence of 

 period between carbonic acid at a temperature over 3000° C, the periods 

 of oscillation of both the incandescent and the cold gas belonging to the 

 extra-red portion of the spectrum. 



It will be seen from the foregoing remarks and experiments how impossible 

 it is to examine the effect of temperature on the transmission of heat, if 

 different sources of heat be employed. Throughout such an examination 

 the same oscillating atoms ought to be retained. The heating of a pla- 

 tinum spiral by an electric current enables us to do this while varying the 

 temperature between the widest possible limits. Their comparative opacity 

 to the extra-red rays shows the general accord of the oscillating periods of 

 our series of vapours with those of the extra-red undulations ; hence, by 

 gradually heating a platinum wire from darkness up to whiteness, we 

 gradually augment the discord between it and our vapours, and must there- 

 fore augment the transparency of the latter. 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 th& radiation from the same spiral at a red heat ; 26 per cent, of 

 the radiation from a white-hot spiral, and only 2 1 per cent, when the spiral 

 is brought near its point of fusion. Remarkable cases of inversion as to 

 transparency occurred in these experiments. 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 nearly 

 fusing 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 

 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 

 molecules of formic ether oscillate as a whole more slowly than those of 

 sulphuric ether. 



When the source of heat was a Leslie's cube filled with boiling water and 

 coated with lampblack, the opacity of formic ether in comparison with 

 sulphuric was very decided ; with this source also the position of chloro- 

 form, as regards iodide of methyl, was inverted. For a white-hot spiral, the 

 absorption of chloroform vapour being 10 per cent., that of iodide of 

 methyl is 1 6 ; with the blackened cube as source, the absorption by chloro- 

 form is 22 per cent., while that by the iodide of methyl is only 19. This 

 inversion is not the result of temperature merely ; for when a platinum wire 

 heated to the temperature of boiling water was employed as a source, the 

 iodide was the most powerful absorbent. Numberless experiments, indeed, 

 prove that from heated lampblack an emission takes place which synchro- 

 nizes in an especial manner with chloroform. This may be thus illustrated. 

 For the Leslie's cube coated with lampblack, the absorption by chloroform 



