The other kind of fermentation is simpler in action, but more incomprehensible in character. It is produced by mere contact, without any change in the ferment itself. Thus it is known to chemists that spongy platinum causes the combination of oxygen and hydrogen, and exerts generally a powerful influence over the affinities of gases and liquids, without ever itself undergoing any change. The influence of Emulsine, in causing, by mere contact, the Amygdaline in the bitter almond to resolve into Prussic acid and other compounds; and that of Pepsin or of Ptyaline, in promoting the change of Starch into Sugar, is of this kind. Considering that Catalytic medicines are not by their nature changing bodies, being mostly minerals, it is not likely that they could cause that kind of fermentation which requires that the ferment itself should be in a state of change. The influence of contact is the one which they would be most calculated to exert. I may remark that I have used the term Catalytic without any reference or allusion to this sense, in which it has been frequently employed, but merely as conveniently expressive of undoing or destroying. I would not wish, either in the terms or in the propositions which I have adopted, to assume for granted any thing which is not proved, still less an idea which is purely hypothetical. But it is not very unlikely that some of these medicines may act in a mode which is more or less analogous to an action of fermentation of the kind just described. They might then either cause change themselves, and by this means alter and destroy a morbid process somewhat similar to that which they themselves excite; or they might, by simple contact, be able to resolve this process into a natural direction. We have seen that when introduced into healthy blood, they invariably tend to produce a change in it which is productive of harm; but that when there is already an abnormal process going on there, their influence will effect the subversion or annihilation of this other process. And there are certain physiological considerations that render such an idea still more intelligible and plausible. It is to be remembered that the blood, in which we suppose such actions to go on, is not an ordinary chemical fluid, subject to common laws, and influences, such as we may meet with out of the body; but a very complicated mixture, which is ever circulating and being maintained at a high temperature, and contains a number of compound organic bodies, each of which is liable to a series of varied metamorphoses. It is not a very potent agency which is needed to disturb the chain of conditions of one of those inconstant bodies which is thus continually performing the circuit of the system. I may briefly exemplify the series of changes, simple but momentous, which an organic body is capable of undergoing, if I instance the combination of elements which constitutes Urea.