SPECIFIC GRAVITY, OR WEIGHT. 



43 



act accordance with their different specific gravities : in 

 other words, light bodies, or those having a small specific 

 gravity, would rest upon, or rise above all heavier bodies, 

 or those possessing a greater specific gravity. 



.. In the case of different liciuids, the particles of which are 



Wliat are illus- , , , , . 



trations of this free to move among themselves, this arrangement always ex- 

 prmciple? jg^ gQ iQ^g ^g ^Jjq different substances do not combine to- 



gether, by the force of chemical attraction, to form a compound substance. 

 Thus, water floats upon sulphuric acid, oil upon water, and alcohol upon oil, 

 and by carefully pouring each of these liquids successively upon the surface 

 of the other, they may be arranged in a glass in layers. 



Carbonic acid gas is heavier than atmospheric air. We accordingly find 

 that it accumulates at the bottom of deep pits, wells, caverns and mines. 



This principle also explains certain phenomena which at 

 first seem opposed to the law of terrestrial gravity, that all 

 matter is attracted toward the center of the earth. We ob- 

 serve a balloon, a soap-bubble, or a cloud of smoke or steam 

 to ascend'; and a cork, or other light body, placed at the bot- 

 tom of a vessel of water, rises through it, and swims on the surface. These 

 phenomena are a direct consequence of gravitation ; the attraction of which, 

 increasing with the quantity of matter, draws down the denser air and water 

 to occupy the place filled by the lighter bodies, which are thus pushed up, 

 and compelled to ascend. 



Why does a 

 balloon ascend, 

 or a cork rise 

 to the surface 

 of water ? 



Fig. 9. 



Suppose a, Fig. 9, a ball of wood so loaded with lead 

 that it wiU float exactly in the middle of a vessel of water. 

 The weight of the wood and the upward pressure of the 

 water have such a relation to each other, that the ball is 

 balanced in this position. If now we add a few drops of 

 strong salt and water, we shall see, as it sinks and mixes 

 with the water, that the ball, a, is forced to the top of the 

 fluid, because the attraction of gravitation on the denser 

 f uid draws it down, and compels it to occupy the place 



The principle that the particles of hquids arrange them- 

 selves according to their specific gravitie.", has been taken 

 advantage of in the West Indies by the slaves, in order to 

 enable them to steal rum from casks. The long neck of a bottle filled with 

 water, is inserted through the bung of the cask into the rum. The water 

 falls out of the bottle into the cask, while the lighter rum rises to take its 



place. 



The principle of specific gravity admits of many valuable 



applications in the art.s. It offers a very sure and quick 

 method of determining whether a substance is pure or adul- 

 terated. Thus, silver may be mixed with gold to a consider- 

 able extent, without changing, to any great degree, the ap- 



Mpntion some 

 of the practical 

 applications of 

 specific gray- 

 ity. 



