The causes of disease have been studied from the very earliest times; the earliest writers on medicine wrote far more about the laws of health than they wrote about medicine. Hippocrates, the celebrated father of medicine, wrote far more about the health of the people, and the prevention of disease, than he did about the cure of disease. He wrote, for instance, works about Foods, about Diet in acute diseases, about the use of Liquids, and his celebrated work about Air, Water, and Places. You see merely from the titles of these works that they more concern the preservation of health and the prevention of disease than medicine proper or the cure of disease, and the great physicians who followed Hippocrates followed his example in this respect.

Now, at the very outset of the subject, we come across certain terms, that are well known to us, that are household words, but which we require to understand more definitely before we can go on to anything else.

We talk about a person's constitution ; what do we mean by a person's constitution ? We say a person has a strong constitution or a weak constitution; what do we mean by it? A person has a strong constitution when he has no tendency to, and no sign of, disease. A person has a weak constitution when he has a marked tendency to disease of any kind ; one marked tendency to disease of any kind is sufficient. Constitution has been defined as the resultant of the physiological forces of the body. But if any one of these forces tends in the wrong direction, the person has no longer a strong constitution. These forces are very many ; one of the most important of them is hereditary tendency. If a person, by hereditary tendency, is liable to a particular kind of disease, that person cannot be said to have a strong constitution. We do not speak of a person that is liable to consumption for instance, one who belongs to a consumptive family, as having a strong constitution; he has a hereditary tendency to that disease. And so the capacity of the lungs, the power of the heart, or, if you like, the power of the circulation, the power of digestion, and a large number of other physiological forces-if I may so use the term-are elements in our constitution; they are the forces from which the resultant, which is our constitution, is made up; but it must be remembered that any one of these forces being weak, or tending in the wrong direction, towards disease rather than health, makes us have a weak constitution instead of a strong one.

Now, persons who originally had a strong constitution, by virtue of their good hereditary tendencies, may turn it into a weak one by following habits of one kind or another, by excesses of one kind or another, originating in that way a tendency to disease, or actually developing diseases of one kind or another, so that persons having originally strong constitutions may ruin them, as it is called.

After dividing people into those who have strong constitutions and those who have weak constitutions, the ancient physicians, with Galen at their head, pointed out that healthy people might be divided into classes according to certain marked characteristics ; one healthy person is quite different from another healthy person, and so persons have been classified according to what are called their temperaments. By temperament we mean the predominance of one of the general systems of the body over the rest; for instance, the circulatory system. Some people have a very powerful circulation, strong hearts, an abundance of rich blood, organs well nourished, well developed; they are not readily attacked by disease, and when attacked they very speedily get well; these persons are said to be of the sanguine temperament. Next we have the lymphatic temperament, in which the absorbent system has undue predominance over the other systems. These persons are described generally as of unsym-metrical build, with a pale, flabby complexion, not florid like persons of the sanguine temperament. Persons of this temperament are found especially in damp unhealthy localities, where they are crowded together, and neither take sufficient exercise nor have a sufficient amount of good food : they are subject to diseases known as scrofulous diseases, which attack the bones and lymphatic glands, and cause those marks that you often see in the necks of unhealthy children. Now this temperament requires to be combated by attention to hygienic conditions, especially attention to conditions of place of residence and to eating and drinking. When this temperament is strongly developed it must not be considered as a condition of health, but as a condition of disease, and persons in this condition have descended from persons subject to scrofulous diseases. . There is a third temperament, of which most of you have heard-viz., the nervous temperament: in this the nervous system has undue predominance over the other systems. There are an immense number of people who are working their nervous system to a much greater extent than it should be worked ; in these people the tendency to over-work, over-excitement, and worry gets worse and worse, and it has long been pointed out by physicians that persons who belong to this nervous temperament in a marked degree have the tendency to get worse and not to get better. And on this I ought to insist a little more, because this temperament is most marked in our time: in these days of express trains and telegraphic messages everything goes on so fast, and the competition of life is so great, that the tendency of the great majority of people is to over-work themselves mentally, and to over-worry themselves about big and little cares. When this once begins, it goes on getting worse. This nervous temperament is associated with nervous diseases, and it has long been observed that persons of a distinctly marked nervous temperament belong to families in which nervous diseases are prevalent, and so it is of the greatest importance for persons who have a tendency to this state of over mental work and excitement to be on their guard against either over-working themselves or over-exciting themselves, and it is perhaps of still greater importance that persons should be on their guard against over-working children who are willing to work. The difference between a child of the nervous temperament and a child who belongs to the lymphatic temperament is extremely marked; and it is of the greatest importance for people who have the training of children to take note of this from the beginning, and rather to check the tendency to over-work of the nervous excitable child, and to stimulate the sluggish, drowsy, lymphatic child. The reverse is commonly the case; and children of the nervous temperament, who would do quite enough or too much work if left alone, are excited to over-work and competition of all kinds to gain prizes, instead of being rather held back on account of their excitable natures and too irritable nervous systems.