134. =Nature of the Subject.=--We now come to a very interesting subject, which is at least intimately connected with Hydrostatics, if it may not be considered a part of it. The principles which have been developed in the chapter on Hydrostatics in relation to liquids are to be applied here to various kinds of substances. And as we proceed you will see that all the phenomena brought to view in this chapter are to be referred to the same cause with those of the previous chapter; viz., the attraction of gravitation. 135. =Specific Gravity Defined.=--Before proceeding with the investigation I will give you the definition of specific gravity. The specific gravity of any substance is its weight as compared with the same bulk or volume of other substances. Water is taken as a standard, and its specific gravity is for convenience called 1. Mercury, then, is said to have a specific gravity of 13.5, for it is thirteen and a half times as heavy as the same volume of water. It is easy to see how the specific gravities of different fluids may be ascertained. One mode, and the most obvious one, is to weigh in a vessel equal quantities of them. In what way the specific gravities of solids are ascertained will be explained in another part of this chapter. 136. =Action of Gravity on Solids in a Liquid.=--The reason that a very heavy substance, as a stone, sinks in water is simply that the earth attracts it more strongly than it does the water, and so drags the stone down through it. If the stone lay upon a bladder filled with water, it would press upon it with the force with which it is attracted by the earth. But where water is not thus confined, the stone thrusts its particles to the one side and the other till it gets to the bottom. It is the attraction of gravity, also, that makes light substances, as wood and cork, rise in water. In this case the water is attracted by the earth more strongly than the wood or cork, and so gets below it, and in so doing pushes the lighter substance up above itself. [Illustration: Fig. 87.]