Prof. Tyndall on Calorescence. 391 



tween them. The annexed curve (fig. 3), which is the mean of 

 several, expresses, with a close approximation to accuracy, the 

 distribution of heat in the spectrum of the electric light from 

 fifty cells of Grove. The space ABCD represents the invisible, 

 while CDE represents the visible radiation. We here see the 

 gradual augmentation of thermal power, from the blue end of 

 the spectrum to the red. But in the region of dark rays beyond 

 the red the curve shoots suddenly upwards in a steep and massive 

 peak, which quite dwarfs by its magnitude the portion of the 

 diagram representing the visible radiation*. 



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 radia- 

 tion in the case of the sun must be less than in the case of the 

 electric light. Experiment, we see, justifies this conclusion ; 

 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. If we 

 cause the beam from the electric lamp to pass through a layer 

 of water of suitable thickness, we place its radiation in approxi- 

 mately the same condition as that of the sun; and on decompo- 

 sing the beam after it has been thus sifted, we obtain a dis- 



* How are we to picture the vibrating atoms which produce the different 

 wave-lengths of the spectrum ? Does the infinity of the latter, between the 

 extreme ends of the spectrum, answer to an infinity of atoms each oscil- 

 lating 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 sound 

 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 (Helm- 

 holtz, Ton-Empfindungen,ip. 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 aether ? and may not, further, a single atom, controlled and 

 jostled as it is in solid bodies by its neighbours, be able to impress upon 

 the aether a motion equivalent 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 same refran- 

 gibility. 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. 



2D2 



