Royal Institution. 24 1 



Hull, and the Kelsea bed not to be above a, as hitherto supposed, 

 but below it, having been forced up through a into its present posi- 

 tion. He also regards the Upper Drift (a) as the equivalent of the 

 Belgian Loess, and the beds b as the equivalent of the Belgian Sables 

 de Campine. 



ROYAL INSTITUTION OF GREAT BRITAIN. 



Jan. 20, 1865. "On Combustion by Invisible Rays." By Pro- 

 fessor Tyndall, F.R.S. &c. 



Passing in review the researches and discoveries of the two 

 Herschels, and the experiments of Melloni, Franz, and Miiller on 

 the dark rays of the sun, the lecturer came to the invisible radia- 

 tion of the electric light, and to the distribution of heat in its 

 spectrum. The instruments made use of were the electric lamp of 

 Duboscq and the linear thermo-electric pile of Melloni. The 

 spectrum was formed by means of lenses and prisms of pure rock- 

 salt. It was equal in width to the length of the row of elements 

 forming the pile ; and the latter being caused to pass through its 

 various colours in succession, and also to search the space right and 

 left of the visible spectrum, the heat falling upon the pile, at every 

 point of its march, was determined by the deflection of an extremely 

 sensitive galvanometer. 



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

 ment from the violet to the red, while in the dark space beyond the 

 red it rose to a maximum. The position of the maximum was about 

 as distant from the extreme red in the one direction, as the green of 

 the spectrum in the opposite one. 



The augmentation of temperature beyond the red in the spectrum 

 of the electric light is sudden and enormous. Representing the 

 thermal intensities by lines of proportional lengths, and erecting 

 these lines as perpendiculars at the places to which they correspond, 

 when we pass beyond the red these perpendiculars suddenly and 

 greatly increase in length, reach a maximum, and then fall somewhat 

 more suddenly on the opposite side of the maximum. When the 

 ends of the perpendiculars are united, the curve beyond the red, re- 

 presenting the obscure radiation, rises in a steep and massive peak, 

 which quite dwarfs by its magnitude the radiation of the luminous 

 portion of the spectrum. 



Interposing suitable substances in the path of the beam, this peak 

 may be in part cut away. Water, in certain thicknesses, does this 

 very effectually. The vapour of water would do the same ; and this 

 fact enables us to account for the difference between the distribution 

 of heat in the solar and in the electric spectrum. The comparative 

 height and steepness of the ultra-red peak, in the case of the electric 

 light, are much greater than in the case of the sun, as shown by the 

 diagram of Professor Miiller. No doubt the reason is, that the 

 eminence corresponding to the position of maximum heat in the solar 

 spectrum has been cut down by the aqueous vapour of our atmo- 

 sphere. Could a solar spectrum be produced beyond the limits of 

 the atmosphere, it would probably show as steep a mountain of in- 



