140 



SPECIFIC GRA VITY. 



and a weight (= w), sufficient to sink the point c to the 

 water line, is placed in the pan. The weight of water dis- 

 placed by the instrument = W + w. The hydrometer is 

 now removed, wiped dry, and placed in the given liquid. 

 A weight (= z), sufficient to sink the hydrometer to c, is 

 placed in the pan. 



Note. A Nicholson's hydrometer may be used as a Fahrenheit's 

 in any liquid which has no chemical action upon the metal of which 

 it is made. Neither of these hydrometers gives results as accurate 

 as those obtained by the methods previously given. 



252. Constant Weight .Hydrometers. A 



hydrometer of constant weight consists of a glass tube neai 



the bottom of which are two bulbs. The lower and smaller 



bulb is loaded with mercury or shot. 



The tube and upper bulb containing air, 



the instrument is lighter than water. 



The point to which it sinks when placed 



in pure water is generally marked zero. 



The tube is graduated above and below 



zero, the graduation being sometimes 



upon a piece of paper placed within the 



tube. As a long stem would be incon- 



venient, it is customary to have two in- 



struments, one having zero near the 



top, for liquids heavier than water; the 



other having zero near the bulb, for 



liquids lighter than water. The scale of graduation is arbi- 



trary, varying with the purpose for which the instrument is 



intended. These instruments are more frequently used to 



determine the degree of concentration or dilution of certain 



FIG. 85. 



