CATALYTIC ACTION, (caraXvatc, dissolution.) The following are a few instances of what has been called catalytic action. Clean surfaces of platinum attract hydrogen and oxygen gases so powerfully, that, without combining with them in any way, it draws them into such close contact with itself, and with one another, that they unite and form water ; platinum black will absorb in this way, without chemical union, 250 times its volume of oxygen. Charcoal newly burned and introduced into a mixture of oxygen and sulphu retted hydrogen, causes their combination with such force, that while it remains chemically unchanged, it nevertheless becomes ignited, and the gases explode. When a pound of starch is digested, and kept simmering for a few days, with six or eight pints of distilled water rendered slightly acid by two or three drachms of sulphuric acid, the sulphuric acid, though itself remaining unchanged, causes the starch to be converted into sugar. A ferment introduced into solutions of sugar will cause the sugar to be decomposed into alcohol and carbonic acid, without itself combining in any way with either. Water is an oxide of hydrogen which is capable of taking another atom of oxygen, and becoming a binoxide, still retaining the form of water ; but a piece of gold put into it will suddenly decompose it without becoming itself oxidized. When chlorate of potass is heated to cause it to give off oxygen, it is found that the gas is much more freely liberated when binoxide of manganese is also present, though chemically it takes no part in the decomposition, nor is itself deoxidized. In all such cases where a body effects chemical changes
in other bodies, itself remaining unchanged, this action of presence is called catalytic action. It has been supposed by some that light, when it acts on salts of silver to produce an invisible impression, modifies the salt in some way to communicate to it catalytic pro perties. The most probable account of such an effect would be, that the chlorine undergoes some modification similar to that which oxygen in certain processes undergoes when it becomes ozone, with out losing its chemical identity. Light has been shown by Professor Schiinbein, in certain cases, to produce this effect on oxygen, and in all cases of catalytic action it may be that the catalyser, if we may use that term, renders allotropic one or more of the elements with which it comes in contact. This notion of the modifying power of the light is not opposed to the theory which supposes that a gradual reduction of metal takes place in the camera, though in such small quantity as to be inappreciable, because the modifying power may be the cause of the reduction. No doubt the development of the image commences at a point which our eyes cannot reach, and it is of no importance whether that commencement takes place in the camera or the dark room.