473 
for ascertaining Specific Gravities . 
their difference ; and on dividing its weight in air 
by this difference the quotient will he its specific gra- 
vity. 
The weight of a body in air is found by putting it in 
the upper cup, and adding grains until the hydrometer 
sinks in water to the mark on the stem. Now, as the 
substance and the additional weights in the cup will be 
altogether three hundred grains, the weight of the body 
will of course be so many grains as the weights put in fell 
short of three hundred. Its weight in water will be 
found by putting it into the lower cup , and adding grains 
in the upper cup until the instrument sinks as before : 
the complement of the weights in the top cup to three 
hundred being in like manner its weight in water. 
Example. 
If a body weighs in air one hundred and twenty grains, 
and in water one hundred and four, the difference is six- 
teen. On dividing one hundred and twenty by sixteen, 
we have for the quotient .75, or (taking, as before, the 
specific gravity of water at one thousand) 7*500 for the 
specific gravity of the body. 
This instrument affords us consequently a very ready 
way of determining the purity or value of any alloy or me- 
tallic ore, and is therefore particularly adapted to the mi- 
neralogist. Thus, for example, the weight of a guinea, 
or its weight in air, is one hundred and twenty-eight 
grains ; and if the gold is of its proper standard, it will 
weigh about one hundred and twenty- one grains in wa- 
ter, or will lose one- eighteenth part only of its weight 
in air. If it loses more, therefore, it is not of its pro- 
per specific gravity, and consequently not of standard 
gold. 
To find the specific gravity of any of the different 
species of wood or other bodies lighter than water 5— 
Vol. ii, 3 N 
