In the fourth zone, the southern temperate zone, including the important British settlements in Australia, Van Diemen's Land, and New Zealand, which is in general so distinguished for its salubrity, phthisis is one of the five most usual affections, the others being dysentery, pneumonia, catarrh, and ophthalmia.

It is still unknown on what climatic conditions the endemic absence of pulmonary tubercle in the above districts depends. Neither the temperature, nor the hygrometric state of the atmosphere, nor the geological conditions of the soil, afford an explanation of the fact. It does not depend on dryness of the air combined with heat; for though the air in Egypt is very dry, in the East Indies it is very damp and moist, and in both localities phthisis is absent, or at least extremely rare. On the other hand, Nubia, Chili, and Lima also possess a very dry atmosphere, and in these countries phthisis is particularly prevalent.

2. Phthisis diminishes decidedly in elevated situations, in consequence of the great rarefaction of the air. The only exceptions to this general rule have been supplied from Abyssinia. Notwithstanding these exceptions, the mass of evidence brought forward seems sufficient to establish that the occurrence of phthisis diminishes with the atmospheric pressure in vertical elevations. We shall scarcely err in attributing this to the greatly increased rarefaction of the air. It does not appear that the formation of tubercle in the system in general diminishes and ceases on lofty mountains; but it does appear that on account of the rarefied state of the atmosphere with diminished pressure, and an absolutely diminished amount of oxygen, the formation of tubercle is impeded in the lungs, a situation in which it is otherwise particularly liable to occur. This may depend either on the diminished quantity of oxygen, which at a height of 12,000 feet is less by more than one-fourth, or it may be caused merely mechanically by the extension of the act of inspiration thus rendered necessary, with greater dilatation of the thorax and bronchi. We agree with Dr. Mühry that the latter is the more probable cause of the exemption alluded to. We cannot see how a diminution of the supply of oxygen to the lungs should be unfavourable to the formation of tubercle. On the other hand, two ethiological facts are in favour of the latter view. First, the theory that the exemption is due to increased expansion of the lung, agrees perfectly with the fact that the summit of the lung is principally or almost solely the seat of tubercles, and with the probability that this is a result of the less degree of dilatation which precisely this part of the lung must experience in consequence of the conical shape of the chest. Secondly, the structure of the thorax in mountaineers who dwell at a great elevation is very remarkable, the chest being very broad, and the lungs highly developed.

In taking advantage, in a hygienic point of view, of the districts exempt from pulmonary tuberculosis, whether the areal or the mountain regions, we should remember that there is a great difference between sending a phthisical patient merely to a warmer and more uniform climate-which relieves only the diseased condition of the lungs, but where the form of disease is not itself endemically absent or rare-and causing him to reside in a climate where the combined etiological sources of his malady do not exist. If we acknowledge the good effects of the climates of Italy, the South of France, the South of Spain, Madeira, &c., on phthisical northerns, we must expect still more beneficial results from those of Egypt, Algiers, the East Indies, and still more, as we may now confidently hope, of the higher mountain regions. Dr. Mühry suggests that as rarefied air acts by producing a greater expansion of the lung, we may infer that in cases presenting a tendency to pulmonary tuberculosis the frequent and constant practice of deep inspirations would be a rational proceeding of easy adoption. But the first place must be awarded to the climatic treatment, and particularly to residence at sufficient heights on suitable mountains. Dr. Mühry supposes that as the proper and powerful effect of an elevated situation commences at 4500' and may increase until we attain a height of 10,000', which heights correspond to average barometric pressures of 23 and 19" respectively, we shall perhaps find 7000' or 8000' to be the most favorable elevation, corresponding to barometric pressures of 21 and 20 inches. Perhaps, however, a less degree of elevation may be found useful to those accustomed to lowland habitations. It is likely that at no distant period Sanatoria may be established in different situations to render mountain climates more available for sanatory purposes.

Suitable localities might be found on the eastern side of the Andes, in Mexico, with their beautiful terraces, valleys, and table-lands; their large, and in part gorgeous, cities, and their mild temperature, which affords an agreeable climate to an elevation of 8000'; or in the Sierra of the Andes of South America, on their east side, or in the midst of them-localities now rendered accessible by the steam navigation of the River Amazon.

The salubrity of a climate depends in general and principally on-1, a moderate and comparatively equable temperature of the air; 2, on a certain dry condition of the soil; 3, in a minor degree, on certain unknown endemi-physical predispositions (we do not now speak of social circumstances) to particular morbid conditions.

The salubrity of different climates, which is thus a product of several factors, is in many respects relative. There is scarcely a climate which is equally healthy to all men, or which is decidedly unfavourable to its indigenous population. In the first place, the salubrity of foreign lands has usually been estimated from the effects of the climate on European visitors. For obvious reasons, the inhabitants of the temperate zone are those most capable of ubiquity, and the frigid zone is comparatively more tolerable to them than the torrid. In the East Indies, the English rulers bave never maintained their ground for three generations; still less have the Dutch done so in Java. On the west coast of Africa, the second generation has never survived. In the somewhat more temperate climate of the West Indies, a Creole population has been able to exist. The greater part of the population derived from the South of Europe, and now naturalized in America within the torrid zone, inhabits, as is well known, the cool mountain districts. The children of Europeans in the East Indies begin to fall away after the fourth year, and the Creoles in the West Indies have in general lost strength and liveliness. Still, in the East Indies, a mixed posterity of the Portuguese is found in later generations; Jews and Armenians also give here, as well as everywhere, proofs of general acclimatization. A remarkable example of acclimatization is afforded by the extension of the Indian people in tropical America, from the torrid base of the Andes to their icy heights—a difference of elevation of 13,000 feet.

In connexion with this question it is to be observed that there are diseases for which the receptivity of strangers is diminished through a prolonged stay in the country, while there are others the receptivity of strangers for which never diminishes. To the former class belong, in the first place, yellow fever, and also disorders depending on plethora and dyspepsia. To the second class belongs the influence of malaria, in respect to which there is no acclimatization; on the contrary, proclivity constantly increases. In addition, there is a third class of forms of disease for which decided receptivity of strangers does not commence until after a continued residence in the land; these are, certain chronic affections of a cachectic nature-for example, lepra, pachydermia, elephantiasis, framboesia (?), impetigines, tendency to gangrene, &c. Lastly, there is a fourth class, consisting of diseases which first appear in the second generation in children; for example, atrophy, arising after the period of dentition. In general, acclimatization for hot countries is to be defined as consisting, in the first place, in an accommo dation of the quantity of blood to the high temperature, as a reduction of the relative plethora with which the northern arrived, and probably of the quantity of fibrin. Two years' residence is, on an average, considered to be necessary to this end. The signs of the change are to be found in the disappearance of the high complexion of the skin, and the substitution for it of a paler and yellower colour. In the West Indies, a diminished receptivity to yellow fever commences with this change. There is no proper so-called acclimatization fever, though every first feverish attack which affects a newcomer has been indefinitely so styled; it is most commonly a malaria fever. When inhabitants of the tropics have lived for some time in colder zones, or in the cool mountain regions of their own country, they likewise acquire a corresponding acclimatization, their blood increasing in quantity and in fibrin; and after their return home they present a more inflammatory tendency, and diminished immunity from the miasm of yellow fever. Europeans inhabiting the tropics experience after a residence in cold countries a more rapid disacclimatization in reference to the torrid zone.

With reference to relative salubrity, it is to be observed, secondly, that a climate may

be only individually healthy or unhealthy. Thus Italy possesses an individual salubrity for patients from the north affected with pulmonary or rheumatic diseases; but the tables of disease and mortality show that, while it has a more beautiful, it has by no means a more healthy climate than northern Europe. Canada, on the contrary, has an unhealthy climate with respect to the diseases just mentioned, but its climate is in itself more healthy than that of Italy.

That a difference exists between the races of men with respect to susceptibility of certain forms of disease, cannot be denied. Negroes possess a very extensive immunity from the malaria intoxication, such as the Indians in Asia and America do not possess; this immunity will in general be found to hold good also in reference to insolation and cerebral affections. On the other hand, they are peculiarly susceptible to cold, and to the contagions of small-pox and of framboesia; the spinal marrow, too, is in them particularly excitable, tetanus is of frequent occurrence, after exposure to cold or after wounds; delirium, however, even in fever, is rare. If they change to colder climates or to more elevated regions, the affections of the respiratory organs predominate as the causes of their mortality-pneumonia, but still more, pulmonary tuberculosis; also typhus, with which at home they are absolutely unacquainted. But it belongs to the class of inexplicable anomalies, that they do not last in some climates situated in the tropical zone-for example, in the Mauritius, and still less in Ceylon; phthisis is the proximate cause of their mortality in these localities. On the other hand, they constitute an increasing and numerous population in the southern States of North America, where, nevertheless, severe winters occur; and they prosper remarkably in the temperate zone of the southern hemisphere, in the states of La Plata. Of the Indians in America we find it remarked, that in New Granada, where goîtres occur in the highest degree combined with cretinism, they are quite free from these affections. This exemption may also be regarded as a peculiarity of race.

Thirdly. We must recognise a more general or partial salubrity or insalubrity. A particular form of disease may prevail to such an extent that it alone makes a particular locality to be avoided; or vice versa, the absence of a particular formidable disease may invest a district with great advantage. Examples of this we have in malaria fever, yellow fever, the plague, goître, malignant carbuncle, Guinea worm, diseases caused by cold (pneumonia, croup, catarrh); or some perfectly endemic forms of disease, as the Aleppo abscess, the Yemen ulcer, Beriberi, plica, &c. Particular examples of such partial salubrity are also afforded by some superficial districts in reference to phthisis, and by others with respect to carcinoma.

If we review the districts in the tropical zone most remarkable for insalubrity, we shall find that, next to the high temperature, moisture is the most important element in producing an unhealthy state. It is a low-lying clayey soil, rich in mould, most frequently alluvial, at the mouths of rivers, or along the coasts or the banks of rivers, and in marshes, which is most pernicious, and which acts almost exclusively by producing malaria. In fact, where the soil is dry, as on sand and coral formations, or in districts free from rain or rivers, the high temperature of the air is much less injurious, so far as the production of malaria is concerned; while other miasmatic diseases, as, for example, yellow fever and cholera, are but slightly connected with the conditions of the soil, and dysentery and hepatitis not at all so. By attending to these points the insalubrity of many districts has been in a great measure overcome. Batavia has for several decennial periods ceased to deserve the epithet of "the grave of the Europeans;" the part of the city built on alluvial soil has been almost deserted as a residence by the Dutch, who now keep only their stores there, but live on higher ground. In Sierra Leone, on the east side of which lies a large undrainable marsh, an improvement of the great insalubrity has been obtained in another mode, namely, by the introduction of better water for drinking, in iron tubes from the heights. Arica, on the coast of Peru, situated in that rainless dry desert, used formerly to become, after the overflow of a small river, a dangerous abode, but has now been rendered healthy by preventing that occurrence. Acapulco, on the west coast of Mexico, has been rendered more healthy since, by cutting through a mountain, a freer draught of air has been obtained, and the drying of the soil has thereby been promoted. In Surinam, and in numberless other places, health

has improved simultaneously with the culture of the soil, as well as after the removal of woods and the drainage of the land,

If we look in Europe for some standards of comparison, we find the mortality in London, 1:40 to 44; at Edinburgh, 1:35; at Geneva, 1:43; at Hamburgh, 1:30; at Berlin, 1:38; at Breslau, 1:26; at Vienna, 1:24. The great difference exhibited by these numbers is probably in a great measure due to the unfavourable effects of the extreme cold of the more eastern countries during the first year of life. In youth, the deaths from phthisis constitute one-seventh of the entire mortality, in many cities their proportion amounts even to one-fifth. In manhood, pneumonia and bronchitis, in old age, apoplexy and paralysis, are the most frequent causes of death. Some well-known large cities afford examples of the importance of different conditions of the soil. Amsterdam lies on a damp clayey soil, and suffers from malaria; Berlin is situated on sand, and is indebted chiefly to it for its salubrity; Vienna and Paris are built on dusty chalk, which is probably one reason of the frequency of pulmonary tubercles in those cities. Munich, again, is situated on an elevated plain exposed to the north wind, and therefore suffers much from pneumonia; St. Petersburgh has in its neighbourhood a low damp soil, and though malaria has here almost attained its geographical limit and is nearly absent, and the mortality is usually given incorrectly and as exaggeratedly unfavourable, still an extraordinary fatality exists in the first year of life.

If we look for particularly healthy districts, small maritime islands in general carry off the palm-for example, the sandy islands in Northern Europe; also the more southern islands-the Azores, Madeira, Rhodes, &c.: for they combine the moderate and uniform sea climate with a dry soil, and are in a better position to prevent the entrance of importable diseases.

The temperate zone of the southern hemisphere is, however, incomparably before all others in point of salubrity. This is proved unexceptionally in Chili, Patagonia, Buenos Ayres, the southern provinces of Brazil, the Cape of Good Hope, Port Natal, South Australia, Van Diemen's Land, and New Zealand. Here there is, on the one hand, absence of the principal zymotic diseases which have not as yet been imported or become endemic; on the other, we have the admitted general conditions of a sanitary state. The soil is free from malaria south of the isothermal line of 68° Fahrenheit; the temperature is moderate, but as it is liable to considerable oscillation, the greatest mortality is derived from this source, and is attributable to diseases of the respiratory organs--pneumonia, phthisis, &c.: in addition, catarrh and ophthalmia constitute the major part of the morbid constitution. Of this zone it is even asserted that the European settlers improve as a race in their posterity.

The subject at present under our consideration resolves itself into three practical questions: 1, How the insalubrity of climates is to be avoided; 2, How it is to be diminished; 3, How the salubrity of climates, especially of such as are singularly healthy, is to be used (climatic therapeutics). .

The chemical constitution of the atmosphere is so generally known that we need not enter into it here. Dr. Mühry divides its non-essential admixtures into two classes, the chemical and the inherent. The first are again subdivided into those which are and those which are not constantly present. Of the former, it is observed that carbonic acid, being capable of absorption by water, will in general be found in less proportion over a damp than over a dry soil, while over the sea scarcely a trace of it will be discoverable. Of the latter, traces of nitric acid are found in rain falling after thunderstorms, probably formed by the electrical discharges from azote and oxygen. Ozone, the separate existence of which is still in some measure problematical, belongs to this class. Other gases are found in particular localities, but are too limited in extent to be of climateric importance. The inherent non-essential admixtures of the atmosphere are small microscopic particles suspended in the air, consisting of the most minute detritus of substances partly mineral, partly vegetable and animal, partly also of the most minute organisms (fungi and vibriones). Our knowledge of such of this class as are ordinary constituents of the atmosphere is as yet too imperfect to enable us to distinguish those which are extraordinary, or to decide whether they are of etiological importance. It is certain that the majority are indifferent to the human organism. While the expression

"development of miasms" is so generally used, as representing the undefined causes of definite forms of disease of a sporadic or epidemic nature, it is frequently not borne in mind, that such a causal connexion has not been demonstrated. As a nosogenetic or peculiar atmospheric miasm, we can properly assume but one, that is, the miasm of influenza; three others we assume, developed in the soil, viz., malaria, the miasms of yellow fever, and of Indian cholera.*

In reference to geographical distribution, we find the atmosphere in its essential constituents homogeneously composed throughout the whole world; but in accidental, gaseous, or inherent admixtures we find some peculiarities over the sea, over the continent, and over particular limited districts of the latter. These peculiarities are connected with the soil, and are partly products of human cultivation, but they are too locally limited to affect the atmosphere to any extent, or too indifferent in themselves to communicate a special quality to a climate. In a geographical point of view there are no zones or areal districts where the proportions of the atmosphere do not possess the general homogeneousness. Hence it follows that the significance of the often-used expression "pure or impure air," is at best but strictly local, and probably referrible chiefly to the peculiar exhalations of crowded human beings, and always more to the lower strata of the atmosphere, especially when in a state of stagnant rest.

In addition to the regular decrease of gravitation from the poles to the equator, irregular local differences have been shown by observations with the pendulum to exist in different localities. These facts prove that in some places either inequalities in the form of the surface, or varying thickness of the internal mass must occur. gravitation increases in the neighbourhood of volcanic regions, and in other places, from The intensity of causes which are not sufficiently understood. Deviations of the pendulum from this cause are observed at the island of St. Helena, Ascension Isle, St. Thomas, and the Isle of France. The intensity of gravitation is very weak at Bordeaux, it increases suddenly at Clermont Ferrand, at Mailand, at Padua, where it attains its maximum, thence extending in the same condition to Parma. It does not appear that these differences, the extreme of which is one-half per cent., have any influence on the human organism. Small islands, where the population is limited, and consequently does not contain so many receptive individuals as to keep up contagion by regeneration, enable us to arrive at conclusions as to the original occurrence of diseases, or the importation of contagion or miasms. On observations made upon them Dr. Mühry bases the following classification:

I. Diseases which do not occur originally or spontaneously, but only after importation, occasionally spreading epidemically, are—

1. The contagious: small-pox, scarlatina, measles, hooping-cough, typhus, plague, pustula maligna; these are regenerated only in the organism.

2. The terrestrial miasmatic: the West Indian yellow fever (hæmogastric fever), East Indian cholera (serogastric fever), they are imported, very probably, germinating in the wood of ships; the third terrestrial miasm, malaria, is not imported.

II. Diseases which very probably are capable of spontaneously generating contagion in the system, are—

Dysentery, ophthalmia (both contagions of the mucous membrane), erysipelas, puerperal metritis, hospital gangrene.

III. Diseases which actually originate in the air: of these there is only one, influenza; it depends on a really atmospheric miasm, which, unknown in its essence, at times arises in the air, and is not imported.

As to the meteorology of small islands, the temperature is more uniform and moister; the proximity of a great continent must certainly, when the wind is on that side, make a difference and bring dryness, heat or cold. The regularity of the sea and land winds, the former by day, the latter by night, is also no small advantage. What has been said of the insular, will also be true of the sea-coast or littoral climate. The following may be enumerated as littoral and insular diseases: yellow fever, which affords but a few,

Investigations of the atmosphere in reference to such etiological particles have been instituted by Ehrenberg, A. Vogel, R. D. Thomson (Appendix to the Report on the Cholera, 1855); (Gazette Medicale de Paris, Oct, 1855), on the occasion of epidemics of cholera.