Specific Gravity. Specific Gravity is the density of the matter of which any body is composed, compared with the density of another body, assumed as the standard, or 1.000. This standard is pure distilled water for liquids and solids, and atmospheric air for gaseous bodies and vapors. In the United States and England the specific gravity, unless when otherwise expressed, is always taken at 60° F.; but in Prance at 32°, or the temperature of melting ice. In most cases, however, it is sufficient merely to note the temperature, and to apply a correction, depending on the known density of water or air, at the different degrees of the thermometric scale. The above plan has been adopted, because the weight of an equal bulk of different substances varies greatly. Thus, as gold is 19 and silver 10 times heavier than water, those numbers, 19 and 10, are said to represent the specific gravity of gold and silver. The heaviest of all known substances is the very hard metal used for making points to the so-called diamond gold pens. It is called iridium ; its specific gravity is 23. Next comes platinum, 21; gold, 19; mercury, 13.5; lead, 11.3; silver, 10; copper, 8; iron, 7; zinc, 6; different kinds of stones, from 4 to 1; aluminum, 2.5. Flax and all woody fibres have a specific gravity of 1.4, and are thus heavier than water, but wood will float or sink according to the number of its pores into which the water does not penetrate. So ebony and many kinds of hard wood sink, pine and all kinds of soft wood float. Cork is the lightest wood, its specific gravity being only 0.24, less than one-quarter that of water. Alcohol is about three-quarters the weight of water, and as the strength of liquor depends on the amount of alcohol it contains, this strength is simply found out by its specific gravity indicated by the more or less floating of a little instrument called a hydrometer, the weaker liquid being little lighter than water has the strongest buoyant power; solutions of different salts, sugar, etc., being heavier than water, have a stronger buoyant power; vessels therefore will draw less water in the sea than in fresh water, and it is more difficult to swim' in the latter than in the sea. The lightest of all liquids has a specific gravity of 0.6; it is called chimogene, and is made from petroleum; it is exceedingly volatile and combustible; in fact, it is a liquefied gas. Carbonic acid gas or choke damp is about 500 times lighter than water; common air, 800; street gas about 2,000, and pure hydrogen, the lightest of all substances, 12,000 times. The heaviest substance has thus 23x12,000 or more than a quarter of a million times more weight than an equal bulk of the lightest ; and the substance of which comets consist, has by astronomers been proved to be even several thousand times lighter than hydrogen gas.

48. To find the Specific Gravity of a Substance heavier or lighter than Water

48. To find the Specific Gravity of a Substance heavier or lighter than Water. In order to ascertain the specific gravity of a body heavier than water, the following method is adopted. First weigh it in air, then weigh it immersed in water. The difference between these two weights will be its loss of weight in water, or, in other words, the weight of the water displaced. Then divide the weight in air by its loss in water, and the result is the specific gravity. Thus, suppose a substance weighs, 12 pounds in air, and 10 pounds in water.

Its loss is 2 pounds in water.

Divide 12 (weight in air) by 2 (loss in water), and the result is its specific gravity,6.— That is, the substance is, bulk for bulk, 6 times as heavy as water.

If the substance will not sink in water, then weight must be added to make it just sink below the surface. This extra weight, added to the weight in air, show its loss in water. Thus, if a substance weighs 8 pounds in air, but requires 2 pounds to be added to submerge it in water, its loss of weight in water is 2 added to 8=10 pounds.

Proceeding as before, we divide its weight in air, 8, by its loss in water, 10 and we have it specific gravity 8/10=.8.