Another instance of these singular transformations may be noticed. The pictures formed on cyanotype paper rendered more sensitive by the addition of corrosive sublimate are blue on a white ground and positive, that is, the lights and shadows are the same as in nature, but, by the application of heat, the colour is changed from blue to brown, from positive to negative; even by keeping in darkness the blue colour is restored, as well as the _positive character_. Sir John attributes this, as in the former instance, to certain rays, which, regarded as rays of heat or light, or of some influence _sui generis_ accompanying the red and orange rays of the spectrum, are also copiously emitted by bodies heated short of redness. He thinks it probable that these invisible parathermic rays are the rays which radiate from molecule to molecule in the interior of bodies, that they determine the discharge of vegetable colours at the boiling temperature, and also the innumerable atomic transformations of organic bodies which take place at the temperature below redness, that they are distinct from those of pure heat, and that they are sufficiently identified by these characters to become legitimate objects of scientific discussion. The calorific and parathermic rays appear to be intimately connected with the discoveries of Messrs. Draper and Moser. Daguerre has shown that the action of light on the iodide of silver renders it capable of condensing the vapour of mercury which adheres to the parts affected by it. Professor Moser of Königsberg has proved that the same effect is produced by the simple contact of bodies, and even by their very near juxtaposition, and that in total darkness as well as in light. This discovery he announced in the following words:—“If a surface has been touched in any particular parts by any body, it acquires the property of precipitating all vapours, and these adhere to it or combine chemically with it on these spots differently from what they do on the untouched parts.” If we write on a plate of glass or any smooth surface whatever with blotting-paper, a brush, or anything else, and then clean it, the characters always reappear if the plate or surface be breathed upon, and the same effect may be produced even on the surface of mercury; nor is absolute contact necessary. If a screen cut in a pattern be held over a polished metallic surface at a small distance, and the whole breathed on, after the vapour has evaporated so that no trace is left on the surface, the pattern comes out when it is breathed on again.