As a general rule, there seems to be the following connection between 'the geological characters of a site and its probable healthiness. Granitic, metamorphic, and trap rocks are usually healthy; there is generally a slope, so that water runs off readily, the air is dry, vegetation is moderate, and drinking-water generally good. They are, however, supposed to be unhealthy when they have become disintegrated, as at Hong-Kong, iuto dark-colored soil. Clay-slate rocks are regarded as healthy, for very similar sea Eons; water, however, is often scarce. Of the varieties of limestone rocks, the hard oolite is the best, and the magnesian (which, if possible, should always he rejected as a site) the worst. Chalk, wheO unmixed with clay, forms a very healthy soil; but if it be mixed with clay, it loses its permeability, and is often damp and (dd. The air is pure, and the water, though hard, is clear, sparkling, and pleasant. The sandstones, if permeable, are healthy; but if, from an admixture or underlying of clay, they lose this property, they are often damp. The water must he carefully exam ined..The bard millstone grits are very healthy. Gravels of any depth are healthy, except water rises through them. Dr. Parkes considers gravel-hillocks as the Manliest of all sites, and the water as being very pure. Gay, dense snarls, and alluvial soils must be regarded with suspicion. Such soils, and especially the deltas of rivers, should, if pos sible, be avoided as sites, and if they must be chosen, thorough subsoil draining. casein] purification of the water, and elevation of the houses far alame the soil are tlat measures to he adopted. According to Dr. Forbes Watson, nearly one-third if the whole surface of India is covered by alluvial soil.
CLIMATE.—The most important climatic conditions connected with the air are tem perature, humidity, and movement, weight, and composition of, the air. Under the head of temperature we might cuter into the general subject of acclimation; we must, however, confine m to the remark, that Europeans from temperate climates see n to flour ish in countries not much hotter than their own, as iu some parts of Australia and New Zealand, although it is vet too soon to decide whether the gsnera: vigor of the race will improve or diminish. in countries with a yearly menu of 20° Palos higher than their home clinfate, as in many parts of India, the race seems to dwindle, and gives indica tions of dying out. The, endenlic diseases of Europeans in the tropics are liver-disease and dysentery, but it is uncertain Low far other influences may be at work besides heat in the production of these diseases. Rapid changes of temperature are always danger ous. '1 he sudden check to the free action of the skin caused by a cold wind, is sure to to catarrh, inflammations, and neuralgia. The registrar-general's returns show that when the temperature in London falls front to 27°, the weekly mortality is inercassd by 400, bronchitis being the disease which mainly causes this increase—an affection which usually does not prove fatal in more than about 40 cases weekly. The fatal influence of extreme cold in depressing the nervous system,-and giving rise to a sleep from which there is no aWakening, is noticed in the article COLD. According to their humidity, climates are also divided into moist and dry. The most agreeable amount of moisture to most persons iswhen the relative humidity* is between 70 and 80 per cent.
In chronic lung diseases, a still moister air is most pleasant, and serves to allay cough. The morbid effects of undue moisture are always associated of temperature. As a general rule, warmth and great humidity are less oppressive than cold and great humidity. There seems to be close relation between the and the checking of certain epidemio diseases and the relative moisture of the atmosphere. The ntala•ious sflseases arc most intense when the moisture is excessive; while plague and small-pox are checked by a very dry atmosphere. Yellow fever seems unaffected by this atmospherio condition. That the humidity of a climate, irrespective of other climatic relations, is not injurious to life, may be inferred from a comparison between the climates of land and Ireland. Tlie number of persons over 100 years of age is. in proportion to the population, five times as great in Ireland as in England, crud the greatest longevity has been observed in Connaught, the wettest of the provinces.f Sec Mapother, op. cit., p. /34. t.
:::The movement of the air is another climatic condition of importance. but it must he considered in connection with heat and moisture. A cold wind abstracts the bodily hest in proem- ion to its velocity; while a hot wind. if dry, increases evaporation, and may thus portly neutralize its own heating power. Variations in atmospheric pressure are of great importance in relation to health. "In mountains," says Dr. Parkes. "there is rarefaction, i.e., lessened pressure of air, lowered temperature, and lessened moisture above 4,000 ft.; greater nmvement of the air; increased amount of light; greater suit-radiation, if clouds are absent; and the air is freer from germs of infusoria. Owing to the rarefaction of the air and watery vapor, there is greater dial hermancy of the air; the soil is, rapidly heated, but radiates also fast, hence very great coolness of the ground and of the air close to it at night."—Op. cit., p. 418. The physiological effects or lessened pressure begin to be perceptible at somewhat less than 3.000 ft., at which altitude the mercury falls 3 inches. The pulse is quickened by 15 or 20 beats, and the breathing by 10 or 15 inspirations per minute; there is increased evaporation from the skin and lungs, while the urinary secretion is probably diminished. At an elevation of 6,000 or 7,000 ft., as in he Swiss Alps, the effect of the monatain air shows itself in a'ma•ked improve ment in (Upsilon, sanguification, and in nervous and muscular vigor. At great heights there is swearing of the superficial vessels, and occasional bleeding of the nose nail lungs; and a sensation of weight is felt in the limbs from the lessened pressure on the joints. A residence for sonic time in a mountain-air is of great value in all anamic affections, from whatever came they may arise. Neuralgia, gout, and rheumatism are all benefited by high alpine positions (see Weber On the Climate tf the Swiss Alps, 1864); and scrofula and consumption a 'e almost absent in the true alpine regions, while ratieuts affected with these diseases, brought.to such a climate, rapidly improve. On the other hand, pneu monia, pleurisy, and acute bronchitis are more common in high regions than lower down. The diJease formerly known as " mountain asthma" seems, 1 rout \Veber's observations, to be common prhnonary emphysema combined with or followed by chronic bronchitis.