COAL 261 
found. This cor- 
responds to the 
weight of the 
water displaced, 
2.é. to the weight 
of a volume of 
water equal in 
bulk to that of 
the substance it- 
self. We have 
now only to di- 
vide the weight 
Fig. 147._Specific Gravity Balance. in air by the 
weight of the 
water displaced in order to find the specifie gravity. 
A rough determination of the specific gravity of a piece 
of Coal gave the following result :— 
Weight in air = ... 52 ozs. 
Weight in water... .. 14.028. 
Weight of volume of water displaced 44 ozs. 
Sg AMO Stee 
44 4°5 
The reasons why such a method of procedure determines 
specific gravity belong to investigations usually undertaken 
under the title of experimental physics, which cannot be 
discussed here. The reasons, however, can be easily 
demonstrated by means of the same apparatus as is here 
employed, and should certainly be made clear to pupils 
betore they are required to investigate the specific gravity 
of any special substance. If determination of specific 
gravity be omitted, the weight of the mineral compared 
with that of an equal volume of water should be ascertained 
by observing that Coal will sink in water. It is for this 
reason a moderately heavy substance. 
Specific gravity = 
