SPECIFIC GRAVITY. 65 
with a delieate 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 (keepiny 
the scales clear of the water) and weigh it again; subtract the 
second weight from the just, tu ascertain the loss by immersion, 
and divide the jivst by the difference obtained ; the result is the 
specific gravity. The loss by immersion is equal to the weight 
of an equal volume of water. The trial should be made ona 
small frayment; two to five grains are best. The specimen 
should be free from impurities and from pores or air-bubbles. 
Tor 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 mens 
wire. On the side of the upright (4) which 
faces the spiral wire, there is a graduated 
mirror, and the readings which give the 
weight of the mineral in and out of 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.¢ and dare 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 
porous 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 
specimen 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 of 
water poured out, is the specific gravity sought. The weight 
of the glass bottle itself is here supposed to be balanced by an 
equivalent weight in the other scale. 
5 

TOUT MIM IMT 

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