than is required in temperate latitudes. The remittent form of malarious fevers, according to Dr. Maclean, "is most prevalent and fatal where high temperatures and malaria act in combination." The same authority also says: "I have, in a great many instances, observed intermittents pass at once into a severe type of the remittent form, apparently from no other cause than the descent from comparatively cool and elevated regions into the heated plains, and this under circumstances where there was no reason to believe that the sufferers had been again exposed to malaria."
In the production and spread of cholera, and yellow fever, excessive heat also has an important share. Both are most intimately associated with decomj)osing filth; both are conveyable by the putrefying stools of the sick, and both flourish most in hot climates and hot seasons. In the case of cholera, the bad effects of extreme heat are abundantly shown by what has been observed in the tropics. "In Bengal," says Dr. Goodeve (Reynolds's "System of Medicine," vol. i., p. 129), "the hot seasons, including the hot, and dry, and rainy seasons, have witnessed the worst epidemics." The most fatal cholera months, in both tropical and temperate countries, are found between April and September. Among European troops in Bombay, according to Dr. Ewart's tables, which cover a period of eight years, more than 50 per cent, of the cases treated in the six months from April to September died, while but little over 19 per cent, of the cases treated in the other six months of the year were fatal. That the disease occurs in winter is undoubted, but that it is much less general and severe is equally true. Yellow fever, though resembling cholera in many ways, is unlike it in this—a temperature of 70° or 72° Fahr. is absolutely requisite to its development and propagation, and the occurrence of frost at once puts an end to its ravages.
Heat also contributes largely toward the production of diarrhoea, its more common form, in recognition of this, taking the name of summer complaint. The disease is far more common and more fatal in hot than in cold climates, and in the summer season of temperate climates than in the winter. Putrefying organic matter, held in suspension in drinking-water, is, according to Dr. Parkes, one of the most common causes of diarrhoea, and this condition of the water is very frequent in summer, and comparatively rare in winter.
In all these instances heat appears to act not so much upon the body itself as upon the various external agencies influencing health. It is to bad conditions what fire is to powder. If the match is withheld, the powder is harmless; but, as the two are liable to come together, we secure safety by removing the magazine. So the presence of filth in water, in soil, and above-ground, may be innocuous by itself, but let the action of heat touch it off into decomposition, and we at once get the effects in the shape of disease. Heat thus aids the pollution of both air and water, keeps alive and active
