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SPECIFIC GRAVITY the day specified, when, through the sale of bonds and the accumulation of surplus revenue etc., gold and silver were once more paid out. The result proved the wisdom of the government and banished all misgiving, for, when 1879 opened, only about 11 or 12 million dollars’ worth of notes were offered for redemption, while the treasury at Washington had in its possession about ten or twelve times the amount in gold in its vaults.  Specif′ic Grav′ity.  See.  Specific Heat, a term used in the physical sciences and in engineering to denote the quantity of heat necessary to raise the temperature of one gram of a substance through one degree centigrade at any given temperature. This quantity of heat is known as the specific heat of that substance at that temperature. If we use the calorie as the unit of heat, an exactly equivalent definition of specific heat is the following: The ratio between the amount of heat required to raise the temperature of any body one degree and that required to raise the temperature of an equal mass of water one degree.

The following is a table of specific heats of some of the more common substances:

See.  Spec′tacles are instruments used for aiding the sight. They are said to have been invented in the 13th century. The first ones were very clumsy, and there was very little improvement until the beginning of the 19th century. Spectacles are worn for strengthening the vision, when it is weakened by old age, or to remedy any natural defect, such as nearsightedness, and also to protect the eye from too strong light. The glasses or lenses used should be made of the best of glass and carefully ground, though sometimes they are made from rock-crystal and called “pebbles.” The glasses should be very carefully selected to accomplish the purpose intended, it being often necessary to fit each eye separately. For short sight or nearsightedness the lens is concave, and the weakest glass that will permit distant objects to be seen should be used. For the weakness of old age, which is farsightedness and is first noticed by one’s holding a book farther off in reading etc., the lens is convex, and the strongest glass should be used.  Spec′trograph.  See.  Spec′troscope.  See.  Spectroscopy is a science which has for its object the determination and description of the various radiations which bodies emit, reflect and absorb. It

is to be carefully noted that spectroscopy does not end in the observation of phenomena, but includes also the description of these phenomena in a manner which is at once the simplest and most complete.

The examination of a body by means of the spectroscope includes four steps. These are the Production of Radiation; the Separation of the different Radiations; the Recording of the Radiations; and the Comparison of the Radiations. We shall consider the subject from these four points of view.

A. What the spectroscope receives and analyses is radiant energy. It follows naturally, therefore, that the first step in examining the spectrum of any body is to make that body a source of radiant energy. In many cases this step has already been performed for us. Thus the fixed stars and the gases in which lightning-discharges occur are already rendered self-luminous. All we can do, therefore, is to observe their spectra; we cannot experiment upon them. Bodies at the surface of our earth can generally be made to radiate energy (1) by heating them, as in the case of a red-hot poker; (2) by passing an electric current through them, as in the case of the Plücker tube; (3) by means of chemical combination, as in the case of the Bunsen flame; or (4) by means of luminescence, of which fluorescence and phosphorescence are examples. As to just how and why these processes cause bodies to emit radiant energy very little is known.



B. Passing now to the second step, namely, the separation of the radiant energy of one wave-length from that of the others, this is the peculiar function of the spectroscope, and is usually accomplished in one of two ways: either by interposing in the path of the ray a prism which impresses upon each ray of different wave length a different direction; or by placing in the path of the rays a diffraction-grating which accomplishes the same thing. (See and .) Of these two methods, the first was introduced by Newton about the middle of the 17th century, and the second by Fraunhofer near the beginning of the 19th century. The ordinary prism-spectroscope, shown in figure 1, is