Cold, in natural philosophy, is the privation, or absence of heat. Its immediate effects on the human body are, contraction of the cutaneous pores, and a temporary ob-struction of insensible perspiration. Hence we perceive what is vul-garly called the "goose skin," and the parts thus affected will not recover their usual elasticity, till the spasm be removed, either by external or internal heat, or by friction, which excites the latter. At present, we shall only treat of the consequences resulting from an excess of cold', having already considered part of this subject under the article Catarrh.
Beneficent Nature has enabled our frail and complicated frame, to support the heat and cold of different climates, with equal facility ; and though man has devised artificial means of defending his body against the action of (-old, or more properly, of retaining" the inbred, or vital heat, yet it often happens that, by exposure to extreme cold, the fingers, ears, toes, etc. are frozen : thus, the natural heat of those parts is reduced to the lowest point consistent with life. If, in such cases, artificial heat be too suddenly applied, a mortification will ensue, and the frost-bitten parts spontaneously separate. Hence they ought to be thawed, either by rubbing them with snow, or immersing them in cold water, and afterwards applying warmth in the most careful and gradual manner; by which they will soon be restored to their usual tone and activity. Indeed (a popular writer justly observes), the great secret, or art, of restoring suspended animation, consists in nicely adjusting the natural and artificial stimuli to the exact tone of the irritable fibre.
As moderate cold produces at first debilitating, and eventually bracing effects on the animal body, it is the roost beneficial temperature in the care of febrile, and such diseases as are not attended with extreme debility; but it should never be follow d by any considerable degree of heat. Sydenham, more than a century ago, pointed out the evils attendant on too much heat in sick-rooms ; he seldom would allow his patients even to lie in bed, and very judiciously directed the rooms to be constantly ventilated with cool air. The great benefit derived from this practice in the small-pox, is now generally acknowledged, and arises chiefly from avoiding the stimulus of heat, after its operation.
The great cold produced by evaporation, observes Dr. Dar-win, is now well understood. I all chemical processes, where aerial or fluid bodies become consolidated, part of the latent heat is pressed out, as in the instant when water freezes, or unites with quicklime. On the contrary, when solid bodies become fluid, or fluid ones become aerial, heatis absorbed by the solution : whence it may be said, in general, that all chemical combinations produce heat, and all chemical solutions generate cold. This should teach the careful gar-dener, not to water tender vegetables in the heat of sun-shine, or in a warm dry wind, lest the hasty evaporation should produce so much cold as to destroy them; an effect that will the more certainly follow, as they have been previously too much stimulated by heat, in consequence of which, the power of life, or irritability, had been already diminished.
When treating on the diseases of plants, Dr. Darwin remarks, that though excessive heat is seldom very injurious to vegetation in this country, yet the defect of Chat element, or in common language, excess of cold, is frequently de-structive to the tender shoots of the ash, and the early blossoms of many fruit-trees, such as apples, pears, apricots, etc. - The blights occasioned by frost, generally happen in the spring, when warm sunny days are succeeded by cold nights, as the living power of the plant has then been previously exhausted by the stimulus of heat, and is therefore less capable of being excited into the actions necessary to vegetable life, by the greatly diminished stimulus of a freezing atmosphere.
In the northern climates of Sweden and Russia, where long sunny days succeed the melting of copious snows, the gardeners are obliged to shelter their wall-trees from the meridian sun, in the vernal months ; an useful precaution, which preserves them from the violent effects of cold in the succeeding night; and, by preventing them from flowering too early, avoids the danger of the vernal frosts. In a similar manner, the destruction of the more succulent parts of vegetables, such as their early shoots, especially when exposed to frosty nights, can only be counteracted by covering them from the descending dews, or rime, by the coping stones .of a wall, or mats of straw.
Having given a short account of the sensible effect of a cold temperature on animal and vegetable life, we shall conclude, with a few remarks connected with the natural history of this elementary power. - The properties of cold seem to be directly opposite to those of heat: the latter increases. the bulk of all bodies; the for mer contracts them; and, while fire tends to dissipate. their sub-stance, cold condenses them, and strengthens their mutual cohesion. But, though cold thus appears, by some of its effects, to be nothing more than the absence or privation of heat, as darkness is only the defect of light, yet cold is probably possessed of another quality, which has induced many to consider it as a substance of a peculiar nature. It is well known, that when a continuance of cold has contracted and condensed bodies to a certain degree, if then its power be increased, instead of progressively lessening their bulk, it enlarges and expands them, so that extreme cold, like heat, swells the substance into which it enters. Thus fluids sensibly contract; in a cold temperature, till the moment they begin to freeze, when they immediately dilate, and occupy more,space than they possessed while in a state of fluidity. Hence, liquor frozen to ice in a close cask, is often known to burst the vessel: when ice is broke on a pond, it swims upon the surface; a certain proof of its being lighter, or of a larger bulk, than an equal quantity of water. This dilatation of fluids, however, is probably owing to a cause very different from that of excessive cold alone ; because the power of freezing may be artificially increased, while the intense-ness of the cold receives no considerable addition ; and, on the contrary, a substance capable of melting ice, will increase the degree of its coldness. Thus, for instance, sal ammoniac mixed with pounded ice, or with snow, melts either of them into water ; and increases their cold to a surprizing degree, as is obvious from the effects of this mixture, in sinking the thermometer. Hence the freezing of fluids cannot be entirely considered as the result of cold, but of some unknown property either in the air or water, which thus mixes with the body, and for a time destroys its fluidity. We cannot, in this place, enter into farther particulars relative to this curious subject; but as there have lately been invented several methods of converting water into ice, which may be of service in domestic economy, we shall. communicate the most easy and least expensive, processes of this kind, under the article ICE.