Page:Popular Science Monthly Volume 5.djvu/416

400 becomes solid. But, ice may be chilled to any attainable degree of cold. Prof. Tyndall reduced its temperature 100°, during which process it shrank in volume and became intensely hard. From the blow of a hammer it broke with a vitreous ring.

We often witness the fact that water will not freeze if in rapid motion, although it be much colder than the freezing-point; but in this case will freeze at the bottom where its motion is retarded, forming what is called "ground, or anchor ice." The sandy bottom beneath swiftly-flowing streams is sometimes frozen solid by radiation of its heat to the cold water flowing over it. But no ice will form at the bottom of a pond or lake if the water be at rest; it then forms upon the surface only. The particles of water, as they become chilled to near the freezing-point, expand, become lighter, and continually rise to the surface, where they solidify, forming a roof of ice. This phenomenon opens a most interesting chapter of physical science, and we will presently recur to it.

The freezing-point of water may be changed by pressure, that is, water under pressure will not solidify at a temperature of 32°; nor is it known how great a degree of cold it can resist if a corresponding degree of pressure be brought to bear upon it. The lowering of the freezing-point of water by pressure is one-seventieth of a degree Fahr. for a whole atmosphere. Under a pressure of several thousand atmospheres, ice has been liquefied at or near the temperature of zero; so



that the freezing-point was zero, instead of 32°. The tendency of pressure is, therefore, to keep water liquid, and to render it so after being frozen. The effect of pressure on a cube of transparent ice is well shown in Fig. 1. It is no longer transparent, but is traversed by hazy lines which come into view as the strain is applied. These hazy