4 BULLETIN 949, U. S. DEPARTMENT OF AGRICULTURE. 



the J-inch screen and retained on the ^-inch screen. This material shall be placed 

 in the cast-iron cylinder of the Deval machine as specified for the standard abrasion 

 test on stone. Six cast-iron spheres 1.875 inches in diameter and weighing approxi- 

 mately 0.95 pound (0.45 kg.), each, shall be placed in the cylinder as an abrasive 

 charge. These spheres are the same as those used in the standard rattler test for 

 paving brick. 



(2) The duration of the test and the rate of rotation shall be the same as specified 

 for the standard test for stone, namely, 10,000 revolutions at a rate of 30 to 33 revolu- 

 tions per minute. At the completion of the test the material shall be taken out and 

 screened over a -j^g-inch mesh sieve. The material retained upon the sieve shall be 



Fig. 2. — Details of Dorry hardness machine. 



washed and dried and the percentage loss by abrasion of the material passing the ^5- 

 inch mesh sieve calculated. 



(3) When the material has a specific gravity below 2.20 a total weight of 4,000 

 grams made up of the four groups of sizes described above, instead of 5,000 grams, 

 shall be used in the abrasion test. 



3. DORRY HARDNESS TEST FOR ROCK. 



(1) A core 25 mm. in diameter and about 10 cm. long shall be cut with the diamond 

 drill from the specimen to be examined. The core should in every case be drilled 

 perpendicular to the bedding plane of the rock. After thoroughly drying, the speci- 

 men shall be inserted in grip of the Dorry machine (fig. 2), leaving about 1 inch pro- 

 jecting from the lower end. The grip shall then be inserted in the sleeve so that the 



