XAiTKAL HI1LO.S01MIY. 205 



search the space right and left of the visible spectrum, the heat fall- 

 ing upon it, at every point of its march, was determined by the deflec- 

 tion of an extremely sensitive galvanometer. 



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

 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 dis- 

 tant 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 sud- 

 denly on the opposite side of the maximum. When the ends of the 

 perpendiculars are united, the curve beyond the red, representing 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 vapor 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 

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

 light, are much greater than in the case of the sun. 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 vapor 

 of our atmosphere. Could a solar spectrum be produced beyond the 

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

 of invisible rays as that exhibited by the electric light, which is prac- 

 tically uninfluenced by atmospheric absorption. 



Having thus demonstrated that a powerful llux of dark rays accom- 

 panies the bright ones of the electric light, the question arises, "Can 

 we separate the one class of rays from the other ? " 



One way of doing this would be to cut off the luminous portion of 

 the decomposed beam by an opaque screen, allowing the non-luminous 

 portion to pass by its edge. VVe might then operate at pleasure upon 

 the latter; reflect it, refract it, concentrate it. Tins would be a 

 perfectly philosophical way of detaching the light from the heat, but 

 with our present means we could not thus obtain a quantity of heat 

 suiHcient to produce the results intended to be exhibited before the 

 conclusion of the discourse. Another plan consists in following up a 

 mode of experiment initiated by Sir William llerschel. He examined 

 the transmission of solar heat through glasses of various colors, 

 through black muslin and other substances, which intercepted a large 

 portion of the solar light. Melloni subsequently discovered that 

 lampblack, and also a kind of black glass, while perfectly opaque to 

 light, transmitted a considerable quantity of radiant heat. In Prof. 

 Tyndall's "Lectures on Heat," given at the Royal Institution in 1862, 

 and since made public, experiments with these bodies are described. 

 It was while conversing with his friend Mr. Warren De la Rue, in the 

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