proceeds, till the whole liquid is of equal temperature. In cooling, the opposite process takes place; the particles, as they become colder at the surface, subside, while others, of higher temperature, supply their place, and this interchange and mixture goes on till the whole body of the liquid becomes as cold as the surface. This remarkable property we have already noticed in speaking of the effect of the waters of the ocean in mitigating the temperature of different climates. Let us now see what would be the consequence if the same laws were to hold without limitation or exception. The cooled particles constantly descending, in virtue of their relative specific gravity, would, when the freezing point was reached, suddenly convert lakes and rivers, and the bed of the ocean itself, into a solid mass of ice, the congelation beginning at the bottom, and quickly spreading upward. Nor, when our deep waters were once frozen, would there be any natural means in existence by which they could be thawed to the bottom, because the heated particles, being the lightest, would constantly float at the top, and the warmth could only be diffused, as it is in solids, by the slower and less equable means of conduction. The experiment has been made, and water has been caused to boil by the application of heat to a vessel partly filled with ice, without thawing the congealed cake below.

Now, this would be attended with many disadvantages. The utility of our seas and lakes, in our own and similar latitudes, would be destroyed as means of commerce and of subsistence; and that element which, by its equal and mild temperature, contributes so essentially to the salubrity of all climates, from the tropics to the polar regions, would serve only to chill the atmosphere, and render even our temperate climates inhospitable.

Let us then attend to the modification of the law by which this inconvenience is provided against. Water continues to contract by the application of cold, till it approaches the freezing point; but here a most remark

able deviation takes place. When it has cooled down to 40°, instead of continuing to contract, it suddenly begins to expand, and it proceeds in this new course, till, at 32°, it becomes ice. The fluid is, therefore, at its greatest density, when its temperature is just 8° above the freezing point; and hence the bottoms of our seas and lakes will be generally found, in winter, not to exceed that extent of coldness.* The coldest water, as it approaches the freezing point, rises to the surface. There the ice is formed, exposed to the first return of a more genial temperature, and ready to dissolve with the earliest influences of a warmer sun.

Another remarkable circumstance, which secures the floating of ice on the surface of the water, is, that in the very act of freezing, a farther expansion takes place. By this operation, the specific gravity of ice becomes less than that of water under any circumstances, and it is thus prevented from sinking to the bottom. Did no expansion take place in the process of congelation, ice would continue to float only so long as the water, on the surface of which it was formed, remained below the temperature of 40°. If the temperature happened to be raised above this point, it would immediately sink, and be overwhelmed, giving rise to various inconveniences, though not of so formidable a nature as those already alluded to.

It is not easy for the most sceptical to avoid the conclusion, that the marked and salutary deviation in this case, from the law by which matter is expanded by heat and contracted by cold, is an arrangement of an intelligent and beneficent Creator. The general rule is followed down to the point where it ceases to be beneficial ; and then, by a sudden and surprising change, the very opposite rule takes place, by which disastrous effects are prevented, and various important advantages are se

* It seems unnecessary to notice some remarkable facts which have lately attracted public attention, that appear somewhat to modify this conclusion, ice having been found formed at the bottom of some deep lakes.

cured. Where could we look for a clearer or more satisfactory proof of wise contrivance?

"We do not know," says Whewell, "how far these laws of expansion are connected with, and depend on, more remote and general properties of this fluid, or of all fluids. But we have no reason to believe, that by whatever means they operate, they are not laws selected from among other laws which might exist, as, in fact, for other fluids, other laws do exist. We have all the evidence which the most remarkable furtherance of important purposes can give us, that they are selected, and selected with a beneficial design."

"

ELEVENTH WEEK-FRIDAY.

II. FROST.-THE EXPANSIVE AND NON-CONDUCTING POWER OF ICE.

OUR attention was yesterday directed to some of the peculiar provisions by which the freezing of water is so modified as to prevent the fatal effects that would ensue were the general law of expansion and contraction which regulates heated bodies, to operate without being arrested and altered. But there are one or two other beneficial operations of frost in our climate, which must not be passed without notice.

The expansive power of water, when passing into ice, has already been stated. This power operates with great force, as has been ascertained by experiment. A familiar instance occurs in the bursting of bottles filled with water or other liquids, when corked up and exposed to its influence. The same power affects the soil, when saturated with moisture, heaving up and separating the particles of earth and gravel. This sometimes acts disadvantageously, by throwing out the plants of young wheat, and by loosening the materials of which our

VOL. I.

* Bridgewater Treatise, p. 85.

Y

roads are composed; but it amply repays these partial inconveniences, by its pulverising effects on tenacious soils. Stiff loams, as they are called, that is, lands chiefly composed of an unctuous clay, though abounding in the vegetative principle, are yet naturally in an unfit state for successful cultivation. Their tenacity prevents the absorption and removal of the superfluous moisture during rainy seasons, and in drought renders the soil so indurated, as to obstruct the free growth of the roots of plants, and the secretion of sap. Now the agriculturist knows how to obviate these disadvantages, by the exposure of this kind of soil to the influence of frost. He ploughs up his land into furrows; and, by thus presenting it to the freezing process, finds that the water mingled with the soil, as it expands in being converted into ice, separates, with irresistible force, the adhesive particles of the clay; and, when again contracted, and rendered liquid by thawing, leaves the earth finely pulverised, and brought into a state well fitted for giving forth its prolific qualities in the ensuing year.

Another beneficial property of frost in the form of ice as well as of snow, is the power it possesses of confining the cold to the surface of the earth. The ice binds up the soil, and being a slow conductor, prevents the severity of the season from injuriously affecting the fibres and roots of the plants which nature has, in general, buried to a sufficient depth for their preservation, with the aid of this wise provision. Even when the ice reaches and envelopes the roots, it seldom materially injures them, because it does not easily descend below the freezing point, which is much higher than the usual temperature of the air in northern winters.

Here, again, we find cause of pious admiration. We do not expect a world of perfection; but the contrary. All climates have their inconveniences and evils: such is the condition of a fallen world; but then these disadvantages are always, in a wonderful manner, guarded, limited, and mitigated. They proceed to a certain point;

but there a Paternal hand interposes; and the sentence is pronounced as distinctly as if it were proclaimed with an audible voice, "Hitherto shalt thou come, and no farther." The obvious intention is discipline, and not destruction. In tropical climates, for example, the heat of a vertical sun, as we have seen, is not permitted to accumulate, by perpetual action on one point, as it would thus become intolerable. That great source of light and warmth is made continually to traverse from tropic to tropic; and when his direct rays would strike too fiercely in his passage there, the clouds collect with their shade, the rising winds fan the air, the cooling and fertilizing rains descend, and thus he moves along in his tempered glory, showering blessings from his wings at the moment when he threatened to scorch and destroy. And a similar arrangement is observable with reference to the opposite extreme of intense cold. The wintry blast seems calculated utterly to exterminate both the vegetable and animal creation; but by a series of deeply excogitated contrivances, the calamity is averted, and life and vigour are preserved in the vegetable world, while comfort and enjoyment are communicated to every thing that lives.

How curious and edifying is the analogy between the works of creation and the operations of Divine grace,— between the revelations of the book of Nature and of the book of Inspiration. When the curse fell on man, it was mitigated by the promise, that "The seed of the woman should bruise the head of the serpent;" when the earth was forbidden to yield him food, except as the fruit of painful toil, that very toil was converted into a source of pleasure and improvement.

Here is compensation; but grace goes far beyond the analogy of nature, for it promises heaven for earth,--the absolute and unalloyed blessedness of immortality, for the turmoils and stinted enjoyments of this mortal life. When the terrestrial paradise was closed against man for ever, his eye was directed, across a rugged and gloomy wilderness, and through a swelling flood, to that bright