4 BULLETIN 347, U. S. DEPARTMENT OF AGRICULTURE. 
in a desiccator, weighed in air, weighed in water at 25° C, immersed 
in water for 96 hours, and again weighed in water. The specimen 
should be suspended by means of a thin silk thread from the hook 
on one of the beam supports about 1J inches above the pan when 
weighing. When weighed immersed in water, the small amount of 
water absorbed by the thread is neglected. It is essential that the 
first weight in water be obtained as quickly as possible in order to 
secure the correct specific gravity of the material. This is especially 
true of very porous rock. In practice it is customary to do this by 
first obtaining the approximate weight of a trial specimen, both in 
air and in water, and of the true test specimen in air. The correct 
weight of the true specimen in water is next calculated as nearly as 
possible by proportion. This weight is then placed on the scale pan 
previous to the immersion of the true specimen in water. 
COMPUTATIONS. 
Calling the weight in air a, the weight in water b, and the weight 
in water after 96 hours' immersion c, 
the specific gravity = —37: ; 
the weight in pounds per cubic foot then = — t-X 62.37 pounds; 
c -b 
the water absorption in pounds per cubic foot = — -^X 62.37. 
VALUE OF TESTS. 
The weight per cubic foot is of value in estimating the weight of 
any given volume, as a cubic yard or carload, of the crushed stone. 
Thus, assuming 50 per cent voids in the crushed material, 
the weight in pounds per cubic yard loose approximately = 
Wt. per solid cu. ft. 
2 
The absorption test is of value in judging the probable lasting 
qualities of the rock under the action of frost, since the presence of 
frost in the stone is likely to promote weakness or even disintegration. 
It may also be of value in estimating the probable error due to absorp- 
tion in the weight of a quantity of stone exposed for some time to the 
weather. 
DEVAL ABRASION TEST. 
OBJECT. 
To determine the per cent of wear and the French coefficient of 
wear. 
EQUIPMENT. 
Deval abrasion machine. 
This machine (see fig. 2) consists essentially of the pulley A 1 or 
worm gear and motor as at present used, driving the shaft BB upon 
which are mounted the four cast-iron cylinders CCCC in such a way 
