CATALYSIS, or CATALYTIC ACTION (Gk iarcabetv, to dissolve). Certain chemical reactions are very markedly accelerated in the presence of substances which emerge from the reaction unaltered. For example, cane sugar is inverted (changed into a mixture of dextrose and levulose). in the presence of acids with a velocity that increased with the strength of the acid. This phenomenon is called catalysis, and the acid is said to be a catalyst. Water, which delays the reaction, is said to be a negative cata lyst. Again, it is possible by the formation of an arc under water to obtain ultramicroscopic suspensions of many of the metals, such as gold, silver or platinum. These suspensions have the remarkable property of decomposing hydrogen peroxide very rapidly into water and oxygen, while they themselves remain un changed. A similar action is shown by cer tain organic substances, the enzymes. In the case both of the enzymes and of the ultrami croscopic colloidal metallic suspensions, the catalytic action is immensely diminished by the presence of very slight quantities of certain poisonous substances such as hydrocyanic acid or hydrogen sulphide. Another interesting ex ample of catalysis is the action of platinum black or sulphur dioxide and oxygen, which unite in its presence to form sulphur trioxide. Water vapor is often a catalytic agent; dry ammonia vapor and dry hydrochloric acid will not unite. This example illustrates the fact that catalytic agents accelerate both halves of a reversible reaction, for dry ammonium chlor ide can be vaporised without dissociating into ammonia and hydrochloric acid. Catalysis is
an extremely common phenomenon in chemis try. Its nature is not well understood; it is agreed that in certain reactions' which appear catalytic the "catalyser* enters into the reac tion and is ultimately regenerated, but this ex planation does not cover all the cases found. In the case of platinum black, which acts as a catalyser in many, reactions, Faraday consid ered that the action was due to the approxima tion of the molecules of absorbed gases. This case, however, has also been explained as due to the formation of intermediate products into which platinum enters. In general, most catal ysis is a surface action, and is accelerated by fine subdivision of the catalyst because of the immense areas thus brought into play. Cer tain substances are known to catalyse them selves. See also ENZYMES.
Mrs. Fulhame,