SPECIFIC GRAVITY. 



65 



with a delicate balance ; next suspend the mineral by a hair, or 

 fibre of silk, or a fine platinum wire, to one of the scales, im- 

 merse it thus suspended in a glass of distilled water (keeping 

 the scales clear of the water) and weigh it again ; subtract the 

 second weight from the fii'st, to ascertafn the loss by immersion, 

 and divide the first by the difference obtained ; the result is the 

 specific gravity. The loss by immersion is equal to the weight 

 oi an equal volume of water. The trial should be made on a 

 small fragment ; two to five grains are best. The specimen 

 should be free from impurities and from pores or air-bubbles. 

 For exact results the temperature of the water should be noted, 

 and an allowance be made for any variation from the height of 

 thirty inches in the barometer. The observation is usually 

 made with the water at a temperature of 60° F. ; 39° *5 F., the 

 temperature of the maximum density of water, is preferable. 



The accompanying figure represents the 

 spiral balance of Jolly, by which the weight 

 is measured by the torsion of a spiral brass 

 wire. On the side of the upright (A) which 

 faces the spiral wire, there is a graduated 

 mirror, and the readings which give the 

 weight oi the mineral in and oat o' water are 

 made by means of an index (at m) connected 

 with the spiral wire; and its exact height, 

 with reference to the graduation, is obtained 

 by noting the coincidence between it and 

 its image as reflected by the graduated mir- 

 ror, c and d are the pans in which the piece 

 of mineral is placed, first in c, the one out 

 of the water, and then in d, that in the 

 water. 



Another process, and one available for 

 vorous as well as compact minerals, is per- 

 formed with a light glass bottle, capable of 

 holding exactly a thousand grains (or any 

 known weight) of distilled water. The 

 ppecimen should be reduced to a coarse pow- 

 der. Pour out a few drops of water from 

 the bottle and weigh it; then add the pow- 

 dered mineral till the water is again to the brim, and reweigh 

 it ; the difference in the two weights, divided by the loss oi 

 water poured out, is the specific gravity sought. The weight 

 of the glass bottle itself is here supposed to be balanced by an 

 squivalent weight in the other scale. 



