given moment the temperature of the whole mass, still in a liquid state, would have arrived at the freezing point; and consequently the whole mass would have been frozen, or become solid, at the same moment. The possibility of such a simultaneous congelation is not merely a philosophical deduction, it sometimes actually occurs. Thus, under certain circumstances, particularly if kept entirely free from agitation, water, still retaining its liquid form, may be cooled down to a point several degrees below that of congelation; when, upon a slight agitation, the whole mass is converted at once into the state of ice.

SECTION VII.

The natural Sources of Water.

FOR the supply of a substance of such immediate necessity to the very existence of man, and of such extensive utility in promoting his comforts, nature has provided the amplest means; all however ultimately derived from that mass of water which has been carried up into the atmosphere by evaporation from the sea: so that if that evaporation were to fail, all forms of animal and vegetable matter, with the exception of those which belong to the ocean itself, would soon perish; for under such circumstances the earth would be deprived of those seasonable showers, without which its vegetable productions could not be sustained; and every spring would soon fail, and every river be dried up: for rivers are in most instances formed by the progressive accumulation of various torrents; and these are produced by that portion of rain which, having fallen upon the ridges and inclined surfaces of hills and mountains, descends more rapidly than the soil can absorb it: and springs result, in a manner that will be hereafter mentioned,

from the accumulation of that portion of the rain which sinks beneath the surface on which it has fallen. But it is evident that if the vegetable world were to perish, the animal world could not long survive.

Nor are the laws by which the moisture, contained in the atmosphere, is precipitated from it in dews or rain, among the least admirable instances of the provision made by nature for a constant supply of the wants of man.

The mechanism, if the term be allowable, by which the formation of clouds and the occasional descent of rain are regulated, resides in the variableness of the state of the heat and electricity of the atmosphere: in consequence of which a given mass of air is incapable of retaining, in solution or suspension, the same quantity of moisture which it did before; and hence that moisture is precipitated in the form of dews and fogs; or, being previously condensed into accumulated masses of clouds, is discharged from those clouds in the form of rain.

It almost seems puerile to illustrate the adaptation of the present laws and order of nature to the wants of man, by the supposition of the consequences that would ensue from a failure of those laws; and yet, as in actual life we often feel not the value of the good which we possess, till admonished by the prospect of its loss; so, with reference to the constitution of nature, we may more forcibly be impressed with the conviction of its general harmony and subserviency to our wants, by the supposition of its being different from what it is, than by the direct contemplation of its actual state. In supposing then that means had not been provided for the regular discharge of portions of that mass of water which has been carried up into the atmosphere by the process of evaporation, the existence of that mass would have been of little avail to man: for mere contact of an atmosphere, however moist, could not promote vegetation

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
Respiration.

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