to any useful extent;* and the formation of springs and rivers would be as effectually prevented by rain ceasing to fall from the atmosphere, as if the material of the rain itself did not exist in it. Of the modes in which nature disposes of the rain that has fallen on the earth, and of the formation of natural springs and rivers, more particular notice will be taken hereafter: but it may be observed by the way, that, although there is scarcely any substance which water is not capable of dissolving to a certain extent, and consequently no natural form of water is pure, yet in almost every instance the natural forms of water are not only innocuous, but salutary. SECTION VIII. The Air of the Atmosphere, as connected with Ir we suppose the atmosphere deprived of heat, and light, and moisture, and of all those other heterogeneous particles which are either naturally or accidentally contained in it; there still remains the medium which is the receptacle or vehicle of those various substances: and this medium is indeed that, which in common apprehension is understood to be the atmosphere itself." Of the vital importance of atmospherical air no formal proof can be required; for every one capable of the least reflection must know that its presence is almost constantly necessary to the existence of man, from the moment of his birth to that of his death. Of all other external aids we may be deprived for a comparatively long time without danger, or even * Niebuhr asserts, what is confirmed by other travellers, that many tracts in Egypt and Palestine, formerly well cultivated and fertile, are at present mere deserts for want of irrigation. (Descript. de l'Arabie, p. 241.) without much inconvenience; of light and heat for instance, and of food and sleep: but we cannot be deprived of the air which we breathe even for a very few minutes, without dreadful distress; or, if for more than a very few minutes, without the extinction of life. This vital importance of the air depends, principally, on its capability of assisting to withdraw from the body, chiefly through the agency of the lungs, portions of that peculiar principle called carbon; the permanent retention of which would be incompatible with the continuance of life. And the union of this principle with one of the constituent parts of atmospherical air is probably effected in the lungs during the process of respiration; the compound passing off in the act of expiration, in the state of an aeriform fluid, called carbonic acid gas. But, in order to give a clear idea of the nature of the process of respiration, it will be necessary to explain more particularly not only the constitution of that portion of the atmosphere which supports this process, but some of its chemical relations to other substances. Atmospherical air then, considering it in its adaptation to the process of respiration, consists of a mixture or combination of two aeriform fluids, which are very different from each other in character, but intimately blended together in the proportion of four to one. Of these two fluids, that which is in the smaller proportion is not only capable of supporting life, when respired or breathed alone; but is capable of supporting it for a much longer period than an equal volume of atmospherical air would have supported it: and if, instead of being employed for the process of respiration, it be made the medium of supporting combustion, the consequent phenomena are still more remarkable; for the combustible body not only burns for a longer time than it would have done in the same quantity of atmospherical air, but it burns with an intensity much more vivid; the light of the flame being in many instances too powerful to be easily borne by the eye. On the other hand, that constituent part of atmospherical air, which is in the greater proportion, not only will not support either life or flame, even for a short time, but extinguishes both, almost in an instant. By numerous experiments, which it is at present unnecessary to describe, it has been ascertained, that many of the metals are capable of attracting and combining with this respirable part of the air: during which process the metallic body assumes an earthy character, and becomes increased in weight; while the weight of the air, in which the experiment has been conducted, becomes diminished exactly to the amount in which that of the metal has been increased: and, at the same time, the residuary portion of the air which has been employed in the experiment equals only about four fifths of the original volume; and is now incapable of supporting either life or flame. But, by processes well known to chemists, the metallic substance may be made to yield a quantity of air equalling that which has been lost during the experiment, the metal at the same time returning to its original state and weight; while the air, thus separated, if added to the residual portion, not only restores the volume and weight of the original quantity; but also its power of supporting life and flame. If, instead of a metal, certain inflammable substances be employed, similar changes are effected on the air; and the inflammable substance, together with an increase of weight and other alterations, acquires acid properties; and hence that respirable portion of the air has, from a Greek derivation, been called oxygen; as being the effective cause of the acidification of those inflammable bodies. It has moreover been ascertained that, during combustion, a piece of pure charcoal weighing twenty-eight grains combines with as much oxygen gas as would weigh seventy-two grains: and, as the volume of the gas employed remains the same at the end of the experiment that it was at the beginning, provided it posed; but also a similarity of custom, with reference to the enjoyment of social intercourse, between communities not less widely separated from each other by time and space, than by degree of civilization; between the luxurious inhabitants of imperial Rome eighteen centuries ago, and the savage tribes of north-western America at the present day. The author of the account of Pompeii states (p. 187— 190,) on the authority of Tooke's Russia, "that the Russian baths, as used by the common people, bear a close resemblance to the vapour bath (laconicum) of the Romans. They usually consist of wooden houses, situated, if possible, by the side of a running stream. In the bath-room is a large vaulted oven, which, when heated, makes the paving stones lying upon it red hot; and adjoining to the oven is a kettle fixed in masonry for the purpose of holding boiling water. In those parts of the country where wood is scarce, the baths sometimes consist of wretched caverns, commonly dug in the earth close to the bank of some river. The heat in the bath-room is usually from 104° to 122° of Fahrenheit; and may be much increased by throwing water on the glowing hot stones in the chamber of the oven. The Russian baths therefore are vapour-baths; and it appears that even the savage tribes of America are not wholly unacquainted with the use of the vapourbath, Lewis and Clarke, in their voyage up the Missouri, have described one of these in the following terms: We observed a vapour-bath, consisting of a hollow square of six or eight feet deep, formed in the river bank by damming up with mud the other three sides, and covering the whole completely, except an aperture about two feet wide at the top. The bathers descend by this hole, taking with them a number of heated stones, and jugs of water; and, after being seated round the room, throw the water on the stones till the steam becomes of a temperature sufficiently high for their purposes." *Sauer, in his account of Billings's expedition, describes the same kind of bath as used in north-western America (p. 175.) It appears then, from the foregoing statement, that the peasants of Russia, and the savages of North America, are in the habit of employing the same means for converting water into vapour, which were employed by the Romans at the most luxurious period in their history: and to the peasants of Russia and the savages of North America, may be added the natives of New Zealand and other islands of the Pacific ocean; merely with this qualification, that they employ the steam, so raised, not for the purpose of a vapour-bath, but of dressing their food. It is worthy of notice, as illustrative of the social feeling inherent in human nature, that, equally among the uncivilized natives of America as among the luxurious inhabitants of ancient Italy, "it is very uncommon for an individual to bathe alone; he is generally accompanied by one, or sometimes several, of his acquaintance: bathing indeed is so essentially a social amusement, that to decline going in to bathe, when invited by a friend, is one of the highest indignities that can be offered to him" (p. 190.) SECTION VI. The Fluidity of Water. FAMILIARIZED as we are to the consequences resulting from that property of water, whereby its particles move so easily among themselves as to yield to the least impulse, provided there be space for yielding, we rarely perhaps meditate on its importance: and yet it is entirely owing to this property that a free communication is capable of being maintained between distant parts of the world by means of the ocean at large, and between different parts of the same country by means of navigable rivers; or by those more than rivals of navigable rivers, artifi-. cial canals.* * It has been stated, on credible authority, that an agent of a great proprietor of canals being incidently asked, during a legal examination, for what purpose he conceived rivers had been |