138 SPECIFIC GRAVITY. 



From 238, determine what these two losses represent. Divide as 

 before. The solid used is called a specific gravity bulb. 



Other methods are sometimes used, but as they depend upon the 

 principles already explained, they need not be set forth here. 

 Some of them will be illustrated in the problems. 



248. To Find the Specific Gravity of Gases. 



The specific gravity of an aeriform body is always found 

 by comparing the weight of equal volumes of the standard 

 (air or hydrogen) and of the given substance. The method 

 is strictly analogous to. the one first given for liquids. The 

 air is removed from the flask with an air-pump an in- 

 strument to be studied soon. The accurate determination 

 of the specific gravity of gases presents many practical dif- 

 ficulties which cannot be considered in this place. 



Note. The weight of any solid or liquid (in grams per cu. cm.) 

 represents its specific gravity. Bodies are commonly weighed in 

 the air. But, in common with all other fluid bodies, the air has 

 weight and therefore ( 238) diminishes the true weight of all bodies 

 thus weighed. This diminution is generally disregarded, but in 

 certain delicate operations it must be carefully considered. 



249. Hydrometers. Instruments, called hydrom- 

 eters or areometers, are made for the more convenient de- 

 termination of specific gravity. They dispense with the 

 use of the balance, an instrument requiring careful hand- 

 ling and preservation. Hydrometers are of two kinds : 

 (1.) Hydrometers of constant volume, as Nicholson's. 

 (2.) Hydrometers of constant weight, as Beaume's. 



250. Nicholson's Hydrometer. Nicholson's 

 hydrometer is a hollow cylinder carrying at its lower end 

 a basket d, heavy enough to keep the apparatus upright 

 when floated on water. At the top of the cylinder is a 

 vertical rod carrying a pan a, for holding weights, etc. 

 The whole apparatus must be lighter than water, so that a 

 certain weight (= W,) must be put into the pan to sink 



