W. N. Walker—Specifie Gravity Balance. 109 
TI].—Watker’s Speciric Gravity Batance For Rocks AND 
MINERALS. 
By Wm. N. Waker, F.G.S., Dundee. 
i aoe determination, by means of the chemist’s balance, of the 
specific gravity of rocks and minerals, now more than ever an 
important element in their classification, is an operation of some 
difficulty. Various instruments of different kinds have been devised 
to obtain the same results more easily and expeditiously, and as 
amongst these there is one I had constructed some time ago and 
of which I have lately had very favourable reports from practical 
workers, perhaps an account of it may interest some of the readers 
of the GroLocicat MaGazine. 
The principle of this machine may be easily seen from the accom- 
panying illustration. 
TOMO tH nm al D) 
fa OOUUTT ON TT 
A B is a lever resting on knife-edges at X and graduated from X 
to B in inches and tenths. W is a weight which can be moved out 
or in on A B to suit the size of the specimen weighed. S is an 
upright with a vertical slot to receive and steady the lever. 
The piece of rock or mineral to be tested is suspended from the 
lever A B by a fine thread and weighed in air. The exact point of 
suspension is noted, suppose it to be Z, the distance from X is there- 
fore XZ. The specimen is then immersed in water as shown in the 
sketch, care being taken to remove by means of a brush any air- 
bubbles that may adhere to it, and reweighed ; suppose the point 
of suspension now to be Y, the distance from X is therefore XY. 
Now since as follows from the properties of a lever, the weights 
in air and water are in inverse proportion to the distances of the 
respective points of suspension from X, and since the amount lost 
by immersion is exactly represented by the difference between the 
latter, it is evident that— 
DON, 
_XY — XZ 
An example may perhaps show this more clearly :—Suppose that 
in weighing in air, Z, the point of suspension, is 10 inches from X, 
and that, on reweighing in water, Y is 15 inches from X, then by 
the above formula :— 
x ° : : 
XY = XZ — = os = 3 = the specific gravity of the specimen. 
the specific gravity of the specimen. 
