through the frozen surface. Now if the density of water continued to increase in a regular progression to the moment of congelation, it would necessarily happen, from the sinking of the particles gradually thus condensed, that at some 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.

SECT. 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 ani

mal 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 re

taining, 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 extent1; and

! 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.)

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.

SECT. VIII.

The Air of the Atmosphere, as connected with Respiration.

If 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 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 com