4 BULLETIN U1, U. S. DEPARTMENT OP 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 1£ 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 = _, ; 



the weight in pounds per cubic foot then = — ^rX 62.37 pounds; 



c -b 

 the water absorption in pounds per cubic foot = — -r X 62.37. 



VALUE OP 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 XZ7 ' 



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. 



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, or 

 worm gear and motor as at present used, driving the shaft BB upon 

 which are mounted the four cast-iron cylinders CCCG in such a way 



