6 BULLETIN 347, U. S. DEPARTMENT OF AGRICULTURE. 



with its specific gravity, in practice the pieces may be so broken 

 as to average from 2 to 2\ inches in diameter with the assurance of 

 obtaining a properly sized sample. After weighing, the sample 

 is placed in one of the cylinders of the Deval machine, the cover 

 bolted on, and the machine given 10,000 revolutions at the rate of 

 30 per minute. At the conclusion of the test the sample is screened 

 on the one-sixteenth inch sieve, the material retained thoroughly 

 washed free from dust, dried in the oven, and again weighed. The 

 difference between this weight and that first obtained represents the 

 wear as determined by test. 



COMPUTATIONS. 



The initial weight in every case being 5,000 grams, and calling the 

 final weight a, 



then the per cent of wear= ' __,■ — X100. 



5,000 



The French coefficient standard is based on the theory that a 



rock of standard excellence does not lose more than 100 grams under 



one-sixteenth inch in size during a test. This is equivalent to 20 



grams per kilogram of rock used, or 2 per cent of the original weight. 



Assuming, then, a coefficient of 20 for such a rock, the French coeffi- 



, , . , nn 20 400 40 



cient of wear lor any material = 20 X tt? = xkt = ~„ „^+ ~t ^ ^ where 

 J WW per cent ot wear 



W equals the loss in weight under one-sixteenth inch in size per 



kilogram of rock used. 



VALUE OF THE TEST. 



In this test the sample is thrown the length of the cylinder twice 

 in each revolution so that the individual stones grind against each 

 other, as well as against the sides of the cylinders. The rock may 

 likewise be broken by the impact, so that the test may be considered 

 as one not only for hardness but for toughness. Values of the per 

 cent of wear run from as low as 1 in certain rare cases to as high as 

 30 or even 40 for some sandstones and limestones. 



This test was standardized in 1908 by the American Society for 

 Testing Materials. 



HARDNESS TEST. 



OBJECT. 



To determine the resistance which a rock offers to the displace- 

 ment of its surface particles by friction. 



EQUIPMENT. 



Diamond core drill. 



The core drill is shown in figure 3, and consists essentially of the 

 steel core tube A, measuring 1 inch inside diameter and 4 inches in 

 length, fastened at its upper end to the hollow shank B. This is 

 held in an ordinary drill press, equipped with a special hollow spindle. 

 Running water is introduced in the upper end of the spindle from a 



