FROST, in physiology, such an excessively cold state of the air as converts watery fluids into ice.
In very cold snowy weather, not only water, but urine, beer, ale, milk, vinegar, and even wine, are either wholly or in part converted into ice, though the last but slowly. As to the freezing of expressed oils, a very intense cold may deprive them of their fluidity, so as to be capable of being cut into portions of any figure; but whether they are convertible into real ice, is not yet determined. In Russia oil freezes much harder than with us, but does not even there become perfect ice. Common annisseed-water, and the like weak spirits, are said to be converted into an imperfect ice in Muscovy; and the strong spirits into a substance like that of oil. When brandy freezes, a liquid part, much stronger than common brandy, retires to the centre of the vessel.
Even solid bodies are liable to be affected by frost: timber is often apparently frozen, and rendered exceedingly difficult to saw. Marble, chalk, and other less solid terrestrial concretions, will be shattered by strong and durable frosts. Metals are contracted by frost: thus, an iron tube, twelve foot long, upon being exposed to the air in a frosty night, lost two lines of its length. On the contrary, it swells or dilates fluids near one tenth of their bulk. Mr Boyle made several experiments with metalline vessels, exceeding thick and strong; which being filled with water close stopped, and exposed to the cold, burst by the expansion of the frozen fluid within them. Trees are frequently
quently burnt up with frost, as with the most excessive heat; and in very strong frosts, walnut-trees, ashes, and even oaks, are sometimes miserably split and cleft, so as to be seen through, and this with a terrible noise, like the explosion of fire arms.
Frost naturally proceeds from the upper parts of bodies downwards; but how deep it will reach in earth or water, is not easily known, because this depth may vary with the degree of coldness in the air, by a longer or shorter duration of the frost, the texture of the earth, the nature of the juices wherewith it is impregnated, the constitution of its more internal parts as to heat and cold, the nature of its effluvia, &c. Mr Boyle, in order to ascertain this depth, after four nights of hard frost, dug in an orchard, where the ground was level and bare, and found the frost had scarce reached three inches and a half; and in a garden nearer the house, only two inches below the surface. Nine or ten successive frosty nights froze the bare ground in the garden six inches and a half deep; and in the orchard, where a wall sheltered it from the south sun, to the depth of eight inches and a half. He also dug in an orchard, near a wall, about a week afterwards, and found the frost to have penetrated to the depth of fourteen inches. In a garden at Moscow, the frost in a hard season only penetrates to two feet: and the utmost effect that capt. James mentions the cold to have had upon the ground of Charlton island, was to freeze it to ten feet deep: whence may appear the different degrees of cold of that island and Russia. And as to the freezing of water at the above-mentioned island, the captain tells us, it does not naturally congeal above the depth of six feet, the rest being by accident. Water also, exposed to the cold air in large vessels, always freezes first at the upper surface, the ice gradually increasing and thickening downwards; for which reason frogs retire in frosty weather to the bottom of ditches: and it is said, that shoals of fish retire in winter to those depths of the sea and rivers, where they are not to be found in summer. Water, like the earth, seems not disposed to receive any very intense degree of cold at a considerable depth or distance from the air. The vast masses of ice found in the northern seas being only many flakes and fragments, which, sliding under each other, are, by the congelation of the intercepted water, cemented together.
In cold countries, the frost proves often fatal to mankind: not only producing cancers, but even death itself. Those who die of it have their hands and feet first seized, till they grow past feeling it; after which the rest of their bodies is so invaded, that they are taken with a drowsiness, which if indulged, they awake no more, but die insensibly. But there is another way whereby it proves mortal, viz. by freezing the abdomen and viscera, which on dissection are found to be mortified and black.
Hoar-Frost, a cold moist vapour, that is drawn up a little way into the air, and in the night falls again on the earth, where it is congealed into icy crystals of various figures. Hoar-frost therefore is nothing but dew, turned into ice by the coldness of the air.