selves 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, artificial canals.*

Rarely also, perhaps, do we meditate on the equally important fact, that, throughout the greater part of the world this element usually exists in a liquid state: and important indeed is that fact; for, of the three states under which it is capable of existing, namely of ice, water, and vapour, if its predominant state had been that of ice or of vapour, philosophers might possibly have conjectured, but the world could never have seen realized, the mighty results of commerce as depending on the art of navigation.

From the same physical character of water, above described, namely its fluidity, manifesting itself actively instead of passively, are daily produced results of equal importance to society, and equally surprising in themselves. Who indeed can adequately describe the advantages derived from water in aiding the powers of mechanism, from the half-decayed and moss-grown wheel that scarcely sets in motion the grinding-stone of the village mill, to the astonishing momentum of the steam engine which kneads a hundred tons of heated iron with as much ease as the hands of the potter knead a lump of clay!

And here, since it is of the utmost importance to mankind that this element should usually exist in a liquid state, let us pause a while to investigate the means employed by nature to prevent its rapid conversion either into vapour or into ice. For although its partial existence in both those states is perhaps eventually as necessary to the general good of the world as its more common state of water, yet, if its sudden or rapid conversion into either were not prevented, great temporary evil would necessarily ensue from our privation of it as a liquid.

It has been already mentioned that the atmosphere constantly holds in solution or suspension' a great body of water, in a state of minute division: but the quantity that can be carried up into the atmosphere by the process of evaporation is limited in two ways; first, by the air's incapability of holding in suspension more than a certain proportion; and secondly, by the restraining effect of the pressure of the atmosphere. But the rapid evaporation of water is also prevented by the comparatively low temperature at which all its natural forms exist, even in tropical latitudes.

It has been stated, on credible authority, that an agent of a great proprietor of canals being incidentally asked, during a legal examination, for what purpose he conceived rivers had been made, answered, "that, no doubt, they were intended to feed canals."

The prevention of the sudden conversion of water into ice depends on a peculiarity in its physical constitution, which is no less remarkable in a simply philosophical point of view, than beneficial in its result to the great bulk of mankind. Water, in common with all other forms of matter, is gradually contracted in its volume by a dimunition of its temperature; and ultimately passes into a solid state. It does not however continue to be condensed to the moment of its congelation, but only to a certain degree of temperature; from whence it begins to expand; and continues to expand till it arrives at the point of congelation.

In this deviation from a general law we find a very beneficial accommodation to the wants of man: for had it been the property of water to become more and more condensed as it approached the point of congelation, one of the consequences would have been that lakes and rivers, instead of becoming gradually frozen from the surface towards their bed, would almost in a moment have become one solid mass of ice: and the evil that would be produced by such an effect may be conjectured, by considering that whenever a long protracted and severe frost has thickened to an unusual extent the superincumbent stratum of ice, the difficulty of breaking through the stratum, in order to arrive at the water beneath, is proportionally increased, and sometimes becomes practically insuperable.

It will be interesting to trace the steps by which this providential law of nature is manifested: and the whole process is easily rendered intelligible to any one who will simply bear in mind these three points, namely, that the average temperature of lakes and rivers is during the heat of summer more or less above the 40th degree of Fahrenheit's scale; that water itself at about the 40th degree is of its greatest density; and that under all common circumstances it freezes, or becomes solid, at the 32d degree. If we suppose then the temperature of a pool or lake to equal at any given moment the 50th degree of Fahrenheit; and a gradual reduction of its temperature to take place from that moment by the effect of a constantly diminishing temperature of the air; under such circumstances the following phenomena would occur. The particles of the water at the surface becoming more condensed, that is heavier, as they become cooler, would sink towards the bottom, and be replaced by the hitherto subjacent particles; which in their turn, undergoing a similar decrease in their temperature and condensation, would consequently subside towards the bottom; till at length the whole mass of water had arrived at the temperature of about 40°. From this point any progressive decrease of temperature would have an expansive effect upon the particles of water near the surface; which, being thus rendered relatively lighter than the particles of the subjacent mass, would not subside; but, remaining on the surface, would continue to be expanded and made still lighter till they had reached the temperature of 32°; at which degree, under ordinary circumstances, they would freeze. But the

coat of ice thus formed would be, in some measure, a barrier to the effect of the colder atmosphere upon the bulk of the water beneath; which consequently would remain for a comparatively longer time in a liquid state; and would be easily procured for general purposes, by making partial openings 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.

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.

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.

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.)

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 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 in

stant.

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