Among diseases that are very largely spread in communities, there is one great class of diseases which do not travel from the places in which they are found; they prevail very largely among the people where they exist, but do not travel from place to place ; they cannot be given by one person to another. Such diseases are- consumption, scrofula, rickets, marsh fevers, rheumatic fever, and a great many others. These diseases go by the name of endemic diseases, from two Greek words meaning in or among the people.

Then there is another great class of diseases which I am going to speak about, which can be taken about by persons suffering from them, and spread by many other ways, and which, under certain circumstances, spread from place to place. Many of them are called epidemic diseases, epidemic meaning upon the people. I shall call them all simply communicable diseases, because communicable is a word which does not imply any theory, as it is quite certain that these diseases may be communicated in some way or another, either directly or indirectly from one person to another, and so are taken from place to place. Sometimes they are called contagious or infectious diseases, but as these words both imply some theory, and as there is great confusion, some diseases being said to be contagious and others infectious, I will not use either of these terms, but we will call them communicable diseases.

The first of these diseases, which is perhaps the best type, is the disease'to which I devoted the whole of last lecture,-there is no doubt that small-pox is a communicable disease. And then there are kindred diseases, such as scarlet fever, diphtheria, measles, whooping cough, typhus fever, and enteric, or typhoid fever. These are diseases of the same kind, although there are differences between them, and we will consider them all together. This set of diseases is characterised in a very peculiar manner. When a person has once had any one of » them, he is very unlikely to have it again. They are diseases of definite durations, divided into well-defined periods, one of the most remarkable of these periods being what is called the period of incubation, and they are accompanied by fever.

After a person has been infected by the poison of one of these diseases, it does not break out at once, but a certain period elapses, which is called the period of incubation, while the disease is, as it were, hatching itself in the person who has been infected; that is a very remarkable fact.

Then there are diseases which have somewhat different characteristics, but still they are communicable from one person to another, and are taken about from place to place. These are the leprosy of ancient times, described in the Bible-whatever disease that was, I think any one who reads the chapters relating to it must see that it was a communicable disease: the plague, of which I hope we are never likely to see anything again, and cholera.

Then there is another class of diseases which are certainly communicable, but which differ from those just described in several ways. These are known as- erysipelas, hospital gangrene (found in over-crowded hospitals), blood-poisoning, and two or three others; also certain diseases which are communicable, and for which special Acts of Parliament have been framed with the view of preventing their spread.

Among the lower animals, also, there are diseases which are communicated from one to the other, and may be communicated from the animals to men. One of these is an exceedingly fatal disease, known as malignant pustule, which, when communicated, is almost always fatal; glanders, found mostly among horses; and, lastly, the disease known as sheep-pox, cow-pox, and horse-pox, which, when communicated to human beings in the way I described in the last lecture, produces a mild disease which goes by the name of vaccinia, which I showed you prevented attacks of smallpox.

These are some of the more important diseases which are communicable. Now, in each case, when one of these diseases is communicated, something, which may be called the poison of the disease, passes from one person to the other, and this poison may be given directly from one person to another, or carried about in various ways. It has been shown over and over again that it can be carried in clothes. This fact, taken by itself, is sufficient to show us that there is some material thing which can be taken by one person and given to another, and when given to that other person, may cause the disease in him. These materials can be carried about in clothes, in water, in air, and various other media. We then come to the conclusion that there are such poisons, and that they are material substances. We have gone a great deal further than this. Secretions containing these poisons have been subjected to various kinds of examination; for instance,-vaccine lymph, used for inoculating persons with vaccine matter, and other blood poisons, have been subjected .to examination, and the conclusion come to is, that the poison is a solid substance, and not a gas, nor dissolved in the liquid secretions that have been examined, but is in the form of solid particles, and can be separated out, leaving the,liquid behind. The solid particles in vaccine matter can be separated out from the fluid, and it has been shown that the solid particles contain infectious matter, and that the liquid in which they are suspended does not. It has also been shown that the poisons of these diseases are not volatile; when liquids are subjected to evaporation,*and the evaporated liquid is caught, that liquid does not contain infected material, but the infected material is in the solid substance remaining behind ; so that you see, by direct scientific experiment in certain of these diseases the character of the poison has been shown; this, however, only up to a certain point.

It has long been observed that the phenomena of these diseases resemble in various ways the phenomena which occur during decomposition or putrefaction of organic substances ; and it has been shown by several observers, notably, you will remember, lately by Professor Tyndall, that putrefaction does not occur unless certain material particles are in the liquid, or get into it from the air.