6 PEOFESSOE TYNDALL ON CALOEESCENCE. 



The sun's rays before reaching the earth have to pass through our atmosphere, where 

 they encounter the atmospheric aqueous vapour, which exercises a powerful absorption 

 on the invisible calorific rays. From this, apart from other considerations, it would 

 follow that the ratio of the invisible to the visible radiation in the case of the sun must 

 be less than in the case of the electric light. Experiment, we see, justifies this conclu- 

 sion ; for, whereas fig. 2 shows the invisible radiation of the sun to be about twice the 

 visible, fig. 3 shows the invisible radiation of the electric light to be nearly eight times 

 the visible. Tf we cause the beam from the electric lamp to pass through a layer of 

 water of suitable thickness, we place its radiation in approximately the same condition 

 as that of the sun ; and on decomposing the beam after it has been thus sifted, we obtain 

 a distribution of heat closely resembling that observed in the solar spectrum. 



Fig. 3. 



B 



Ted. blue 



Spectrum of electric light. 



Does the infinity of the latter, between the extreme ends of the spectrum, answer to an infinity of atoms each 

 oscillating at a single rate? or are we not to figure the atoms as virtually capable of oscillating at different rates 

 at the same time ? When a soimd and its octave are propagated through the same mass of air, the resultant 

 motion of the air is the algebraic sum of the two separate motions impressed upon it. The ear decomposes this 

 motion into its two components (Heimholtz, Ton-Empfindungen, p. 54) ; still we cannot here figure certain 

 particles of the air occupied in the propagation of the one sound, and certain other particles in the propagation 

 of the other. May not what is true of the air be true of the ether? and may not, further, a single atom, con- 

 trolled and jostled as it is in solid bodies by its neighbours, be able to impress upon the ether a motion equiva- 

 lent to the sum of the motions of several atoms each oscillating at one rate ? 



It is perhaps worthy of remark, that there appears to be a definite rate of vibration for all solid bodies 

 having the same temperature, at which the vis viva of their atoms is a maximum. If, instead of the electric 

 light, we examine the lime-light, or a platinum wire raised to incandescence by an electric current, we find the 

 apex of the curve of distribution (B, fig. 3) corresponding throughout to very nearly, if not exactly, the samc^ 

 refrangibilitj-. There seems, therefore, to exist one special rate at which the atoms of heated solids oscillate 

 with greater energy than at any other rate — a non-visual period, which lies about as far from the extreme red 

 of the spectrum on the invisible side as the commencement of the green on the visible one. 



