The effect of HEAT is to stimulate the organic functions of the body, but when considerable heat is applied for some time together its effect is to cause languor and lassitude, want of energy, a disinclination for exertion both bodily and mental; it has a depressing effect generally upon the animal functions or the nervous system, and there are some forms of disease that are distinctly traceable to heat as a cause. We all know the effect of hot weather in causing perspiration, and when the operation of high temperature is continued for some time it has a marked influence upon the liver, increasing the quantity of bile that is secreted, and altering its sensible qualities; this is sometimes followed by inflammation of the liver. In this country those attacks of vomiting and diarrhœa which are so common towards the latter end of summer or in autumn are the effects of a succession of hot days. In tropical climates the morbific effects of external heat are still more conspicuous, tending to violent disorders of the stomach and intestines, and also to acute inflammation of the liver and to acute abscesses in that organ. In these cases the heated atmosphere unduly stimulating the secreting function of the liver creates the predisposition to the disease, while the exciting cause of the inflammation may be exposure to cold. There may be deleterious effects from exposure to cold where the climate is quite hot. For instance a man may after the heat occasioned by the employments of the day, undress and lie opposite a window, his shirt wet with perspiration, to enjoy the sea breeze at night, and though the thermometer may be as high as 80° he may have a sensation of cold. If there is real chilliness it may be deleterious. Heat sometimes acts as an _exciting_ cause of disease—it produces sunstroke, or it may produce an eruptive disease such as prickly heat, &c. The effect of extreme COLD (I use the term cold in the popular acceptation), when its application is continued, is that of a sedative upon the organic functions. Though at first causing pain in the extremities, if continued it causes sleep or overpowering drowsiness. Before this complete stupor comes on there may be a blunting of the sensations and confusing of the intellect, giving to the person exposed to it, the appearance of one intoxicated. When persons in this state are suffered to sleep, and the operation of the cold continues, they become less and less sensible to external impressions until death closes the scene. But the effect of cold upon the body within certain limits of intensity and duration is that of a tonic. When its refrigerating and sedative properties can be sufficiently counteracted by exercise and warm clothing, cold is stimulating, refreshing, and invigorating to mind and body, it clears and sharpens the faculties, bestows alacrity and cheerfulness of spirits, and may become a curative agent.