FRAUNIIOFER LINES.—Certain dark lines in the solar spectrum discovered by Fraun hofer. If a ray of light be admitted through a narrow slit in a dark room, and this slit examined through a prism of flint glass, besides the spectrum band of colors, a number of lines will be observed delicately traced parallel to the edge of the prism, and at unequal distances from each other. Fraunhofer distinguishes the most remarkable of these lines by the letters A, a, B, C D, E, b, F, G and H. A is at the beginning of the red ray, B in the middle, and C at the boundary between it and the orange, E in the green, F in the blue, G in the indigo, and H in the violet ; a is in the red and b in the green. Since Fraunhofer's time, however, some three thousand or more of these lines have been discriminated, some being fixed in position, and others are variable. The latter are often termed telluric or atmospheric lines, as it is probable that they are due to the absorption of the air. The study of these lines has led to a method of analysis termed spectrum analysis, to which photography has been essentially useful. (See also Spectrum.)
FREEZING.—The transformation of a liquid into a solid by the influence of cold. A liquid always solidifies at the same temperature, which is termed its freezing point, and the solid returns to its liquid state at the same temperature, the freezing point and the melting point being always the same for each substance.
The freezing point of water or the melting point of ice is 32 degs. Fahr., and is taken for one of the fixed points in thermometrical calculations. The freezing point of mercury is 39 degs. below zero, of sulphuric ether 46 degs. below zero, and of alcohol 203 degs. below zero Fahren heit. By increasing the pressure upon water or upon any substance which expands when freez ing, its freezing point will be lowered, but with such substances as wax, sulphur, paraffin, etc., which contract in freezing, the freezing point is raised by increasing the pressure.