nutricious. When the putrefactive process bas commenced, and flesh has passed into that state commonly called high, it is unwholesome, and entirely unfit for human food. CHAPTER V. OF THE COMPOSITION, AND CONSTANT RENOVATION OF THE ATMOSPHERE. THE air, or the earth's atmosphere, consists of a metamorphosis of the various substances of the earth, which, having abandoned their previously solid and visible forms, exist as elastic and invisible fluids. The most fixed of the fossils are capable, either alone, or in combination, of mounting up and forming a part of the atmosphere; such is known to be the case even with gold. Thus the atmosphere appears to have been produced by the earth; and it is constantly undergoing consumption from changes in matter, always progressing. This waste seems to be balanced by a perpetual generation of new air in the interior of the earth. Air, or the gases which form it, appears to be continually passing up from great depths in the earth, and is probably given off at all depths as well as at its surface. The substances of the earth are perpetually changing from one state into that of another, and during that process a part evidently goes to the formation of the atmosphere. Air is constantly and perceptibly escaping with Air volcanoes. the waters of many springs; this is particularly remarkable in the Western States of Ohio, where it sometimes rushes up with such force as to throw the water entirely out of the springs. The same phenomenon may be observed, in less vigorous activity, in many wells in this country, as at Bath and Buxton. It also makes its escape from the waters of many lakes, coming up apparently with springs, through their beds. I have seen the water of the lake Agnano, near Naples, in a state of vigorous ebullition from this cause, nearly opposite the Grotto del Cano; where also air constantly escapes through the earth, mixed with much carbonic acid gas. The same occurs at the lake Ansanto, also in the kingdom of Naples. Air is perpetually evolved, very profusely, at the air or mud volcanoes; which are apt to occur about the bases of the fire volcanoes. Air volcanoes exist in Italy and Sicily; in Crimea, and the island of Taman and Java. Humboldt also mentions others in New Andalusia and Trinidad. In these volcanoes, as they are called, the air makes its escape through mud, evidently produced by the perpetual stirring of the wet clay soil, by the constant passage of air, which, by volcanic agency, is being largely generated beneath it; and sometimes so profusely, as to occasion eruptions, during which mud and stones are projected upwards, to the height of two or three hundred feet, with loud explosions. There are also caverns from which, during the warmer seasons of the year, air rushes with great force: as at Mount Eoto, near Turin. The air evolved in these various ways, coming newly from the earth, is not at once favourable for animal respiration, or the same as the rest of the atmosphere, but becomes like it by the natural changes and combinations, or mixtures which subsequently take place. That of the air volcanoes contains much sulphuretted hydrogen, or mephitic gas. The passage of solid substances into the aerial form is greatly favoured by a high temperature; so that the constitution of the air must much depend on the prevailing degree of heat, the nature of the soil, and other circumstances of any particular locality. Udiometry has nevertheless failed in the detection of any remarkable difference between the atmospheres of the most elevated and salubrious situations, and that which circulates in those which are unhealthy and in crowded cities. There is nothing in the least surprising in this circumstance; for it cannot reasonably be expected that a small portion of the atmosphere, an invisible and impalpable fluid, can be taken in a glass tube or jar, and shown, by any process we are capable of exercising, to contain anything deleterious, unless it be so far vitiated as to be unfit for respiration and incapable of supporting life; which assuredly must be the case if the general atmosphere of cities were so deteriorated as to afford any appreciable result by such means. However, the feelings and the opposite effects of the air of different regions afford indubitable proof of great variety in its qualities. When we reflect on the immense quantity of air which is constantly employed by cutaneous and pulmonary respiration, in which it is brought into Purification of air by sufficiently close contact with the blood to enable it to absorb some of its constituent parts, and that such absorption, during life, is constantly going on; and that if the atmosphere contain any one of its ingredients in injurious amount, an excess of it, if soluble, like putrid mephitic gas, will be absorbed by the blood, and entering into the circulation, its poison applied to, and deposited in, every part of the body, it must be sufficiently evident that the health is greatly dependant on the respiration of what is commonly called pure air; that is, an atmosphere containing all its materials in such proportion as to be most favourable to animal life. If living beings be surrounded by, and continue in, an atmosphere thus vitiated by the vapour of matters in deleterious excess, their poison is in this way collected and accumulated within the body, so as to occasion disease and death. Such a condition of the atmosphere constitutes one of the chief sources of disease. It appears that as vegetation prepares the earth for animal alimentation, so it also prepares the air vegetables. for animal respiration, and consequently that neither can be fitted for the use of man, and the higher classes of animals, which has not undergone that change which vegetation effects. It is, when the vegetable world is clothed in its vernal foliage, that the delicious purity and fragrance of the air, invigorates and renovates the vitality of the animal kingdom. But even at other times, and when most of the trees have no leaves, vegetation evidently operates a great effect upon the air. These circumstances are of vast importance, and have not been sufficiently well understood. All history shows that whenever vegetation has been suddenly and extensively destroyed, plagues and pestilences have ever been wont to occur. This shows the necessity of the encouragement of vegetation in and about populous places. The sea and all waters, unless putrid, have also a purifying effect on the air; not by absorption at the surface, but by carrying aloft, with the vapours arising from them, the excess of mephitic gas in the atmosphere. Stagnant or putrid waters, which are so from holding mephitic vapour in solution, accordingly pollute the surrounding air by their vapours, which arise surcharged with mephitic gas. The extreme purity and clearness of the air of Paris, and some other cities on the Continent, appears, in a considerable degree, to be owing to the great number of trees in and around them. The greater purity and clearness of the air is also remarkably conspicuous in all woody places about London. CHAPTER VI. OF THE INFLUENCE OF LIGHT ON THE HEALTH, AND ON THE DEVELOPEMENT OF THE HUMAN BODY. VEGETABLES from which light has been excluded, although they may have had plenty of air, are found to be deficient in their natural colour, firmness, |