222 VARIOUS PHENOMENA. SECT. XXIV. 



Bed and orange coloured media, smoked glass, and all bodies 

 that transmit or absorb the hot rays freely, leave strong impres- 

 sions on a plate of copper, whether they be in contact or one- 

 eighth of an inch above it. Heat must be concerned in this, for 

 a solar spectrum concentrated by a lens was thrown on a polished 

 plate of copper, and kept on the same spot by a heliostat for two 

 or three hours : when exposed to mercurial vapour, a film of the 

 vapour covered the plate where the diffused light which always 

 accompanies the solar spectrum had fallen. On the obscure 

 space occupied by the maximum heating power of Sir William 

 Herschel, and also on the great heat spot in the thermic spectrum 

 of Sir John Herschel, the condensation of the mercury was so 

 thick that it stood out a distinct white spot on the plate, while 

 over the whole space that had been under the visible spectrum 

 the quantity of vapour was much less than that which covered 

 the other parts, affording distinct evidence of a negative effect in 

 the luminous spectrum and of the power of the hot rays, which 

 is not always confined to the surface of the metal, since in many 

 instances the impressions penetrated 'to a considerable depth 

 below it, and consequently were permanent. 



Several of these singular effects appear to be owing to the 

 mutual action of molecules in contact while in a different state, 

 whether of electricity or temperature : others clearly point at 

 some unknown influence exerted between surfaces at a distance, 

 and affecting their molecular structure : possibly it may be the 

 parathermic rays, which have a peculiar chemical action even in 

 total darkness. In the last experiment the effect is certainly 

 produced by the positive portion of one of those remarkable 

 antagonist principles which characterise the solar spectrum. 



Thus it appears that the prism resolves the pure white sunbeam 

 into three superposed spectra, each varying in refrangibility and 

 intensity throughout its whole length ; the visible part is over- 

 lapped at one end by the chemical or photographic rays, and at 

 the other by the thermic, but the two latter so much exceed the 

 visible part, that the linear dimensions of the three the luminous, 

 thermic, and photographic are in proportion to the numbers 

 25, 42-10, and 55*10, so that the whole solar spectrum is twice 

 as long as its visible part. The two extremities exert a de- 

 cided antagonist energy. The least refrangible luminous rays 

 obliterate the action of the photographic rays, while the latter 



