contract and expand to a definite mass by cold and heat, a similar definite mass of lead will contract and expand three times more in its mass than iron.

Having conducted these few preliminary experiments, we may now proceed to investigate the extraordinary exception that water presents, to this general law of contraction by cold and expansion by heat.

Provide a Florence flask, with a long neck, yet capable of admitting the bulb of a delicate mercurial thermometer; pour water at the temperature of 80 degrees into the flask, until about two inches of its neck remain unoccupied ; place the thermometer with its bulb dipping below the surface of the water, that the scale may rest upon the mouth of the flask; then make a mark, as already directed, on the glass at the level of the water, and lastly, lift the flask into a quart jar, containing a mixture of ice and salt sufficient to surround the bulb.

As the water cools from 80 degrees, it will contract and fall in the neck of the flask, like the spirit in a former experiment;-let the thermometer be closely observed all this time; the water will continue to contract until it attains the temperature of 40 degrees, and at that moment the water, instead of continuing to contract, will slowly expand, and rise in the neck of the flask, until the thermometer indicates a cold of 32 degrees, and then the water will congeal or freeze; mark the level of this newly-formed ice upon the neck of the flask, then immediately withdraw it from the cold. mixture, wipe it dry, and place it on a ring, as already directed.

This is a remarkable experiment, and the knowledge that emanates from its accurate investigation is of great importance ;—the water, when it contracts by cold to the

temperature of 40 degrees, is said by the chemist to attain its maximum of density ;-he means by this expression, that if it be cooled below 40 degrees, or heated above 40 degrees, that its contraction and expansion I will be similar.

Suppose, for example, the ice in the flask of the last experiment be allowed to thaw, the thermometer after some hours will indicate when the temperature of the water has risen from 32 degrees to 40 degrees; or has attained its maximum of density; as it becomes warmer it will expand, and upon attaining the temperature of 48 degrees, it will rise to the same mark, or be of the same bulk as it was at the cold of 32 degrees-or in other words, at 8 degrees above 40, the maximum of density, or at 8 degrees below, water is of the same bulk.

With this incontrovertible fact in chemistry we are enabled to venture upon an interpretation of the transition of water from the liquid to the solid state during the natural cold of Winter.

If water, like other congealable liquids, continued to contract in bulk, and to increase in density until it froze, large bodies of water, instead of only being superficially frozen in Winter, would become solid masses of ice. Let us select a fresh water lake as an example ;-in Winter, the earth, as already stated, (page 77,) is warmer than the air; the heat is accordingly withdrawn from the surface of the water by the cold breezes that blow over it; the water, thus cooled, increases in density and falls, whilst warmer water ascends, to undergo a similar abstraction of heat in its turn; and these descending and ascending, or convective currents, continue, until the entire bulk of the water has attained the temperature of

40 degrees,its maximum of density and temperature perfectly congenial to aquatic beings.

The cold breezes still blow over the surface of the lake, still abstracting heat, but instead of causing the water to contract or become heavier, as it would every other liquid, it actually causes it to expand and become lighter, so that cold water fairly floats upon warm water, with no more power of sinking through it than a film of oil would have.

This stratum of light cold water cannot communicate depression of temperature to the warmer water beneath, because it has no conducting power, and convective power is out of the question; it therefore remains floating until cooled by the breezes to 32 degrees; at this temperature it has parted with so much heat, that it can no longer remain liquid, and therefore, becomes a solid sheet of ice.

Over this newly-formed surface the winds may continue to blow, with their greatest intensity of winter cold, but it shields the liquid water upon which it reposes from their chilling agency, and preserves it immutably at the temperature of 40 degrees, whatever may be the cold of the ambient air, for ice is a nonconductor.

The temperature of the surface of the ice may fall many degrees below 32 degrees, but the water beneath remains at 40 degrees; accordingly fishes dwell securely in these warm liquid depths until the heat of spring dissolves the icy shield, and then they rise upon the surface of the emancipated waters.

Had the Creator ordained that water should thoroughly cool to its freezing point, by direct conduction of cold from the air, then on the first approach of Win

ter it would congeal into one entire and solid mass of ice, to the destruction of aquatic beings.

But the Creator, in his Power and Goodness, has conferred upon water habitudes with cold and heat, perfectly distinct from those of other liquids; it expands at an immutable degree of cold, and therefore the resulting ice, though solid as a stone, is lighter than the liquid. water upon which it floats and protects from congelation; thus, "By the breath of God frost is given, and the breadth of the waters is straightened ;" thus, "the waters are hid as with a stone;" thus are they "clothed with ice as with a breast-plate," to call forth our admiration and gratitude towards God, even in the season of Winter.

This miraculous transition of flowing water into solid ice, is therefore not an abrupt or sudden phenomenon; if it were, vast and incalculable derangement of the economy and order of the whole Creation would inevitably ensue on the contrary, it ensues with a degree of regularity and slowness, invariably proportionate to the depth of the water, exposed to the cold blasts of Winter.

Considerable time, therefore, elapses before the entire bulk of water is cooled to 40 degrees by convective currents; and until it be so cooled, and its expansion by continued cold effected, it will not freeze.

Thus the chemist discovers, that some deep lakes are never frozen, because the cold of the ambient air has been unable to establish the full agency of convection throughout the vast bulk of their waters, even during the most severe and protracted winter; again, when a sheet of ice has formed upon more shallow waters, it increases its mass with extreme slowness, and rarely gains a thickness at all proportionate to their main bulk; it is, comparatively speaking, but a film; because ice is inca

pable of conducting cold with facility from its upper to its lower surface.

Rivers and streams resist congelation longer than lakes and pools, because their tides and currents interfere materially with the cooling influence of the air; and the water of the ocean in temperate climates, yet more effectually resists such change, on account of its enormous bulk, extreme depth, and large quantity of saline matter; this alone prevents it from freezing until the temperature falls to 27 degrees, or 5 degrees below the freezing point of fresh water.

Within the arctic circle the surface of the ocean is seldom warmer than 27 degrees, and therefore in the decline of Summer it soon attains this temperature, and congelation proceeds so rapidly, that in the space of a few hours, a sheet of ice of an inch in thickness is formed; then the cold continuing, this increases, until it attains a thickness of several feet, and covers the waters with an impenetrable barrier.

An experiment can be made to prove that saline matter present in water, causes it to resist congelation at 32 degrees; add common salt to half a pint of water, as long as it will dissolve; this is indicated, when after stirring or shaking the solution, a very considerable quantity of the solid salt falls or precipitates to the bottom of the vessel in which it is contained; pour off the liquid, and, if necessary, let it run through a piece of fine muslin to render it clear; it is strong brine, or chemically speaking, a saturated solution of chloride of sodium.

Select a bottle that will hold all, or part of this solution, and another that will hold an equal quantity or bulk of fresh water; place these bottles in a mixture of