but carbonate of lime, upon their courses, and substances that are immersed in them; for carbonate of lime, although insoluble in pure water, is soluble if it contain excess of carbonic acid; and as this escapes by exposure to the air, the carbonate of lime falls, as an insoluble solid or petrifaction; this can be imitated by the artificial operations of the chemist, but he is ignorant of the natural process by which the phenomenon of the solution is effected.

Let us now return to the more immediate object of our inquiry, viz., the contraction and expansion of solids, liquids, and air, by cold and by heat.

The chemist, by conducting experiments similar in principle to those described, but with refined apparatus, and with his utmost skill in manipulation, arrives at the conclusion that air, and all aëriform substances, gases, and vapors, when cooled or heated out of the contact of the liquids from whence they were educed or produced, contract alike, and expand alike; but as regards solids and liquids, he discovers that they have distinct contractions by cold, and expansions by heat.

Or in other words, if a definite volume of air at common temperature, i. e., 62 degrees, contract into a less volume by the cold of iced water, and expand into a greater volume by the heat of boiling water, a similar definite volume of chlorine, hydrogen, or carbonic acid, will do precisely the same; if, on the other hand, a definite bulk of liquid mercury contract into a less bulk by the cold of iced water, and expand into a greater bulk by the heat of boiling water, a similar definite bulk of alcohol, submitted to such variations of temperature, will contract six times, and expand six times more in its bulk than mercury; and lastly, if a definite mass of iron

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 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

ice and salt; they are thus exposed to the same degree of cold, and the water will freeze at 32 degrees, but the brine will remain liquid at this temperature, and many degrees below it, as the introduction of a thermometer will prove.

The chemist, by discovering that saline matter present in water thus causes it to resist congelation, is led to consider the ocean as being an enormous magazine of heat; and that it enacts an important part, in yielding heat, for the mitigation of the inland cold of Winter.

Several curious and instructive experiments can be made regarding the phenomenon of the congelation of fresh water; in the first place, provide a cylindrical glass about two inches in diameter, and fifteen inches high, open at one end and closed at the other by a broad "welt," or foot, upon which it will stand firmly.

Into this glass, pour water at 40 degrees, until within four inches of its brim, then introduce a few lumps of clean ice which will float upon the water, and at the same time raise its level coincident with that of the brim of the glass; allow this arrangement to remain for a few minutes that the ice may slowly thaw.

Whilst this thawing is proceeding, detach the delicate thermometer from the brass column of the " hygrometer," page 175, and to its upper part attach a slender string about eighteen inches long, and by this carefully allow the thermometer to descend, through the water, to the bottom of the cylindrical glass, and note the temperature, probably 40 degrees, that is indicated.

Then gradually draw the string, to raise the bulb of the thermometer into the water resulting from the thawing of the ice; and its entrance into this peculiar stratum of water will be known by its indicating a temperature