It is also probable that chemical decomposition is produced by the mere juxtaposition of different bodies. Iodide of gold or silver, perfectly pure, has been placed upon a plate of glass, and a plate of copper covered with mercury suspended over it: a gradual decomposition of those salts is said to have been observed, iodide of mercury to be formed, and the gold or silver salts reduced to a finely divided metallic state.[132] A body whose powers of radiating heat are low, being brought near another whose radiating powers are more extensive, will, in the course of a short time, undergo such an amount of molecular disturbance as will effect a complete change in the arrangement of its surface, and an impression of the body having the highest radiating powers will be made upon the other. This impression is dormant, but may be developed under the influence of vapour, or of oxidation.[133] A body, such as charcoal, of low conducting power, being placed near another, such as copper, which is a good conductor, will, in a very short time, produce, in like manner, an impression of itself upon the metal plate. Thus any two bodies, whose conducting or radiating powers are dissimilar, being brought near each other, will occasion a molecular disturbance, or impress the one with the image of the other. However small the difference may be, an effect is perceived, and that of the most extraordinary kind, giving rise to the production of actual images upon each surface exposed. It is thus that a print on paper may be copied on metal, by merely suspending it near a well-polished plate of silver or copper for a few days. The white and black lines radiate very differently; consequently an effect is produced on the bright metal in the parts corresponding to the black lines, dissimilar to that which takes place opposite to the white portions of the paper; and, on the application of vapour, a true image of the one is found impressed upon the other.[134] Bodies which are in different electrical states act upon each other in an analogous manner. Thus arsenic, which is highly electro-negative, will, when placed near a piece of electro-positive copper, readily impart to its surface an impression of itself, and so in like manner will other bodies if in unlike conditions. Every substance physically different (it signifies not whether as it regards colour, chemical composition, mechanical structure, calorific condition, or electrical state,) has a power of radiation by which a sensible change can be produced in a body differently constituted.