Page:Popular Science Monthly Volume 26.djvu/553

Rh concerning this curious and still little-known fact: "Not every year, but times enough, out on the open sea, I have seen ice come rapidly up to the surface. If the weather is calm, we can perceive, as far out as we can see, small cakes in the shape of a plate, coming from the bottom, rise to the surface. The edge is in the air, but, when the upper part of the plate gets above the level of the water, the plate turns over and lies flat upon the liquid. It is a dangerous business, for a boat may thus in a few minutes be surrounded by immense masses of new ice."

Aside from this anomaly, the formation of isolated blocks of ice in the open sea is very rare. Water of ordinary salinity becomes denser as it cools, for it freezes at about 28° Fahr., and, as we have explained, attains its maximum density at about 35° only if we keep it artificially in the liquid condition. Water that has lost its caloric in contact with the atmosphere soon sinks; sometimes, as Scoresby attests, ice which is formed at medium depth rises to the surface, while sounding thermometers indicate temperatures near or even below the point of congelation at the bottom. Otto Petterssen is of the opinion that, if water submitted to a cold of a few degrees below its freezing-point does not solidify, it is because immobility favors surfusion, or rather, what is very possible, because we do not know all the laws of nature.

Mr. Petterssen has succeeded by a series of experiments in explaining a variety of phenomena which manifest themselves in the boreal seas, and which Arctic explorers have long been acquainted with, without understanding the reason of them. Sea-water, after its passage to the solid state, has not the same chemical composition as before; but besides this change, which we shall speak of again, it has another interesting peculiarity. If the temperature is very low, the ice of the ocean, like nearly all known bodies, contracts by cold; but at a few degrees below the freezing-point, and before melting, it diminishes in volume when heated, and dilates on cooling. Between 14° Fahr. and -4°, according to the age and source of the block, there is produced a minimum of density, the mass acquiring its maximum volume—that is, the behavior of the solid is the inverse of that of river-water.

While it contracts by heating at about 18° or 23° Fahr., the ice of salt-water loses some of the properties which it possesses at lower temperature, and which are common to it with ordinary ice. It has no longer the vitreous aspect, the fragility, and the homogeneity of solid ice, but becomes softer, more plastic, and less transparent; its fracture is less distinct, and cracks and holes multiply in it. And, when brackish water congeals, it loses its disagreeable taste, but its bad looks and want of limpidity deprive it of commercial value.

Sea-water is a very complex saline solution; chemical analysis discovers in it halogen radicals, simple, as chlorine and bromine, or