THE other remaining diseases which may be in part or altogether prevented are the so-called Zyrnotic. The majority of these are contagious, and all have a specific cause. Our knowledge has advanced so far as to enable us to say that, for the spread of contagious diseases generally, four things are essential lst. The contagious or zymotic substance; be this germ or seed, animalcule or vegetable, or a merely chemical compound. 2nd. A condition of atmosphere or of surroundings permitting the life

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... r stability of the chemical constitution of the contagious or zymotic substance. 3rd. A vehicle for the conveyance of the contagious or zymotic principle to the individual, or rather to the part of the individual from which it can enter his system. , 4th. An individual to receive the contagious or zymotic substance whose body is in such a condition that in it the contagious substance-germ, seed, animalcule, or mere chemical compound-can multiply itself. Our knowledge enables us to state, moreover, that any one of these conditions may be so modified by art as to favour or stay the spread of contagion. As special diseases, the zymotic or contagious substance oj which is certainly always about us, spread at times easily, rapidly, and to a vast number-i. e., become epidemic,-and then, having reached a certain degree of prevalence, shrink tc their former dimensions, one must admit either that changing conditions of atmosphere or of surroundings give increased activity to the contagious or zymotic substance, or that usually they destroy or weaken the contagious matter and now fail to destroy its life or stability of constitution ; or that external conditions preceding or present have so modified the system of individuals as to render them more susceptible to the action of some one special zymotic substance. With reference to the spread of zymotic diseases. 1. While we know nothing of the nature of the contagious or zymotic substance of any one of them, we do know that it is present in a special state of activity in different excreta in different diseases : thus, in the gastro-intestinal secretions in typhoid fever, cholera, and cattle plague; in the skin and throat secretions in scarlet fever; in the throat secretion in diphtheria; in the pustular matter secreted by the skin in small-pox ; in the discharge from wounds in pyemia. 2. We know that special conditions of their surroundings have a great effect in favouring the spread of certain zymotic diseases. Thus a temperature of 32° destroys the poison of yellow fever; free dilution with fresh air renders the contagious matter of typhus innocuous. 3. Vehicle. It is now placed beyond question by the labours of Dr. Snow, confirmed as they have been by experience, that water is one of the great agents in diffusing cholera; and, if possible, it has been more unequivocally proved that typhoid fever is carried from individual to individual in the same way.