1865.] On the Invisible Radiation of the Electric Li<jht. 33 



are formed, the most prolific field of inquiry is promised by the study of 

 the drop- and babble-size as a means of proximate chemical diagnosis*. It 

 does not appear that bubble-volume is at present likely to afford an addi- 

 tional equation for gas-analysis ; but we have seen that both drop-size 

 and bubble-size may offer very valuable criteria as to the constitution of 

 liquids, Ajid although the former (drop-size), especially in the case 

 SL t L 2 , is by far the most sensitive to variation in the chemical constitution 

 and proportion of mixed constituents, the latter has the advantage of 

 requiring a much less amount of liquid, and of being applicable to every 

 liquid without regard to its solubility in other liquids. 



II. " Note on the Invisible Radiation of the Electric Light." By 

 JOHN TYNDALL, F.R.S. Received January 13, 18G5. 



Pending the preparation of my complete memoir, which may occupy me 

 for some time to come, I would ask permission of the Royal Society to lay 

 before the Fellows a brief and partial summary of the results of my experi- 

 ments on the invisible radiation of the electric light. 



The distribution of heat in the spectrum of the electric light was exa- 

 mined by means of the linear thermo-electric pile, applied to the sol.-ir 

 spectrum by Melloni, Franz, Miiller, and others. The electric spectrum 

 was formed by lenses and prisms of pure rock-salt, its width being equal 

 to the length of the row of elements forming the pile. The latter, standing 

 at right angles to the length of the spectrum, was caused to pass through 

 its various colours in succession, and to search the spaces beyond the region 

 of colour, in both directions. 



As in the case of the solar spectrum, the heat was found to augment 

 from the violet to the red, while the maximum heating effect was observed 

 beyond the red, and at a distance from the red, in one direction, equal to 

 that of the green of the spectrum in the other. 



The augmentation of temperature beyond the red in the case of the 

 electric light is sudden and enormous. Plotting from a datum line the 

 thermal intensity of the various portions of the spectrum, the ordinates 

 suddenly increase in length beyond the red, reach a maximum, and then 

 fall somewhat more suddenly on the other side. When the ends of the 

 ordinates are united, the curve beyond the red rises in a steep and massive 

 peak, which quite dwarfs the luminous portion of the spectrum. 



The comparative height and steepness of this peak are much greater than 

 those obtained by Professor Miiller for the solar spectrum. Aqueous 

 vapour acts powerfully upon the invisible rays ; and doubtless the action 

 of this substance in our atmosphere has toned down the eminence beyond 

 the red in Professor Muller's diagram. A solar spectrum, produced beyond 



* Some word is required to denote the acquirement of the knowledge of the constitu- 

 tion of a substance without taking it to pieces (analysis). " Diagnosis," used in its purely 

 etymological sense, answers this purpose. 



VOL. XIV. D 



