It has been said that if twenty persons undergo hardship and exposure from shipwreck, the effect of the wet and cold may be in one to cause catarrh, in another rheumatism, in a third pleurisy, in a fourth opthalmia, in another inflammation of the bowels, and fifteen may escape without any illness at all. A predisposing cause is defined to be anything whatever, which has had such an influence on the body as to have rendered it unusually susceptible to the exciting cause of the particular disease. In most cases the distinction is obvious, but it is sometimes difficult to say of a given cause whether it ought to be ranked among the predisposing or the exciting causes. Disease is often warded off notwithstanding the presence of the exciting cause, when we ascertain and prevent the predisposing cause of it, and it may sometimes be averted in despite of strong predisposition, if we know and can guard against the agencies by which it is capable of being excited. When we enumerate causes of disease we see among them many that under ordinary circumstances minister to life, health, and enjoyment; and I can hardly refer at all to the varying circumstances under which they become the medium of pain, disease and death. These circumstances are so various, so many of them are apt to be put in operation at the same time, and so little power have we of excluding them one after the other, so as to ascertain the exact efficiency of each, that our observation respecting their actual effects are open to much fallacy. We cannot for instance in a given case estimate accurately the effect of impurities in the atmosphere such as organic and inorganic dust, nor the effect of differences in degree of its natural qualities such as extremes of heat and cold, sudden variations of temperature, excessive moisture or dryness, different electric conditions, differences of pressure, a deficiency of light, and the amount of ozone, &c. OF HEAT AND COLD AS EXTERNAL AGENCIES CAUSING DISEASE. The range of temperature compatible with human life is very great; men live in the hottest and the coldest climates, where the earth produces any sustenance for them. It requires more care to preserve life under intense cold than under intense heat. Tropical climates are thickly peopled where the thermometer ranges from 80° to 100° for a long time together. In arctic countries on the other hand where the thermometer sinks to 40° or 50° below zero, we still find inhabitants, but they are few and thinly scattered. It is probable that at a degree of temperature a little greater than that of the equator or a little less than that of the poles men would perish. Man is capable of existing under certain circumstances for a short time, and enduring a much higher degree of heat than the general atmosphere attains in the hottest portions of the earth, but there are generally some deleterious effects from hot climates or continued hot weather.