Red and orange coloured media, smoked glass, and all bodies that transmit or absorb the hot rays freely, leave strong impressions 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.