§ 34. DETEEMINATION OF SPECIFIC GEAVITY. 23 



CHAPTER II. 



ANALYTICAL MANIPULATION. 



Determination of Specific Gravity. 



34. By the specific gravity of a solid or Jiquid is un- 

 derstood its weight as compared with the weight of an 

 equal volume of water. 



a. The most obvious method of determining it is to 

 weigh equal volumes of the substance and of water. This 

 is easily accomplished in the case of liquids, with the aid 

 of the so-called specific-gravity bottle or piknometer, an 

 instrument made of thin glass and provided with an ac- 

 curat-ely ground stopper ; the stopper is sometimes per- 

 forated. The weight of the empty bottle is ascertained, 

 then its weight when completely filled with water, or 

 filled to a mark on the neck, and finally when filled to 

 the same extent with the liquid under examination ; be- 

 fore weighing, in each case, all adhering particles of liquid 

 should be carefully wiped off with blotting paper ; both 

 weighings should be made at as nearly the same tempera- 

 ture as possible, or at about 15° C.,the usual temperature 

 of the working room. Divide the weight of the liquid 

 by that of the water,, for the specific gravity of the former. 



h. The specific gravity of liquids is also determined 

 with great facility, though with less accuracy, by means 

 of the areometer or hydrometer ; this is a glass tube 

 closed at both ends, considerably enlarged towards one 

 end, and loaded with mercury to make it take a vertical 

 position in the liquid, but not with enough to cause it to 

 sink under the surface. The use of the areometer depends 

 upon the principle, that the less the specific gravity of a 

 liquid is, the less its buoyant power. The specific gravi- 



