This term, in its most general sense, implies the unequal distribution of a substance at the boundary between two heterogeneous phases, and when these concern a solid and a gas it is usually referred to as gas adsorption. Practically, it means the removal or abstraction of a constituent of gases or liquids by certain agents, as, for istance, the removal of iodine from a solution of it in potassium odide by means of charcoal, or, again, the surface action illustrated by the condensation of hydroqen gas by means of palladium, and the retention of the dark colouring matter of crude sugar solutions, as effected by charcoal during filtration processes. The adsorption compounds, as those of charcual and caramel, or charcoal and litmus, do not resemble ordinary chemical compounds in constancy of composition and more or less resistance to decomposition by physical agencies, and are generally formed at the surfaces, the quantity being proportional to the active surface. The relative adsorptive powers of various kinds of charcoal depend not merely upon their respective capacities to absorb gases, but also to retain them at reduced pressures. It has been ascertained that adsorbents can be prepared from certain colloidal solutions, and silica gel prepared from a colloidal solution of silicic acid is stated to exhibit a power of adsorption equal or superior to that of animal charcoal. It can be prepared from silicate of s&dium by the action of a dilute mineral acid, and is stable in the a:r even at high temperatures. Suphur dioxide at a concentration of 05 per cent, in air, and with a time contact of o8 seccnd, is stated to be adsorbed to the extent of 100 per cent, and it is thought in America, where the process has originated, that this new material will prove very valuable in a number of industrial operations for effecting the selective separation of mixed gases and the saving of solvents.
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