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the material is destroyed. The final blow is easily ascertained, for 

 when the hammer falls on the plunger, if the material beneath it 

 withstands the blow, the plunger rebounds ; if not, the plunger stays 

 at the point to which it is driven, which is recorded on the slip of 

 paper. The automatic record thus obtained from each briquette is 

 filed for future reference. The number of blows required to break 

 the bond of cementation, as described above, is taken as representing 

 the cementing value of the rock, and is so used in comparing this 

 property in road materials. Five briquettes are tested from each 

 sample and their average taken for the result. Briquettes can be 

 seen in the exhibit case before and after they have been subjected to 

 this test; also records of results. 



RECEMENTATION TEST. 



The fragments of the five briquettes that are broken are again passed 

 through a 0. 25 mm. screen, made into briquettes, and broken again to 

 determine the recernentation value. This property is in a way more 

 important than the cementing value, for the dust on a road surface is 

 continually being receinented. With most materials about 50 per 

 cent of the cementing value is lost on recementation, but with a few 

 it increases. 



OTHER TESTS. 



Besides the tests enumerated above, the character of each sample 

 is studied, and each rock is properly classified according to its miner- 

 alogical composition. 



During the present fiscal year a number of new tests will be taken up. 

 Among the most important is a test for determining the toughness of 

 rock, which will be made in the following manner: 



A machine similar in all respects to the present machine used for 

 breaking the rock-dust briquettes will be used, except that it will have 

 a spherical end plunger instead of the present flat one, and cubes of 

 rock will be substituted for the briquette. The test will consist in a 

 drop of the hammer of 1 cm. for the first blow, and an increased drop 

 of 1 cm. for each succeeding blow, until the cube is destroyed. The 

 entire energy of each blow is thus concentrated at the center of the 

 upper surface of the cube, splitting the cube from this point after the 

 elastic limit is reached. The number of blows required to destroy a 

 cube will be used to indicate the toughness of a rock. It can be seen 

 that this treatment approximates very closely the blows of horses' 

 feet and the wheels of vehicles to which road materials are subjected. 



TESTS ON GRAVEL. 



The only test made 011 gravel at the present time is a determination 

 of its cementing value, which is made by grinding 500 grams of a 



