262 



MINERALOGY 



lower pan P'. There should be no air bubbles sticking to it ; the 

 beaker is now raised until equilibrium is established, with the lower 

 pan at the same depth in water as before, when the third reading 

 is taken = 11.1. To determine the specific gravity: 



G = 



the weight in air = W _ 16.1 - 2.8 _ 13.3 

 loss of weight in water W w 16.1 11.1 5 



= 2.66. 



The specific gravity of pure quartz is 2.653. In this method 

 the specimen used should weigh between two and three grams. 

 The chemical balance may be substituted for the Joly balance and 

 accurate weighings made, with a resulting increase in accuracy. 



II. Pycnometer method. The pycnometer, Fig. 375, is a small 

 flask of usually 10 cc. capacity, having a nicely fitted ground 



glass stopper with a capillary bore 

 running through it. The weight of 

 distilled water which exactly fills the 

 pycnometer at 20 C. is determined 

 once for all = C. In each case the 

 pycnometer is weighed empty and 

 dry = P. The specimen is powdered 

 to avoid any internal cavities and 

 about two and one half grams are 

 placed in the pycnometer, it having 

 been dried at 100 C. when there is 

 no danger of loss at that temperature, 

 and carefully weighed = S. The pyc- 

 nometer is now filled about half full 

 with distilled water and very carefully 

 boiled for a few minutes to expel any 



air that may be included in the powder. After cooling, the flask 

 is filled with recently boiled distilled water, care being taken that 

 none of the sample is lost and that the pycnometer is filled to the 

 top of the small capillary bore when the stopper is in place, and 

 cooled to 20 C., when the whole is weighed = D. S = sample 

 and pycnometer weighed together. 



S P = W = weight of sample in air. 

 (S + C) D = loss of weight in water. 



FIG. 375. Pycnometer. 



G = 



S-P 



(S + C) - D' 



