330 THE POPULAR SCIENCE MONTHLY 



Toughness as here understood is the power possessed by a material to resist 

 fracture by impact. The test piece is a cylindrical rock core similar to that 

 used in determining hardness, and the test is made with an impact machine 

 constructed on the principle of a pile driver. The blow is delivered by a hammer 

 weighing 2 kg. which is raised by a sprocket chain and released automatically 

 by a concentric electro-magnet. The test consists of 1 cm. fall of the hammer 

 for the first blow and an increased fall of 1 cm. for each succeeding blow until 

 failure of the test piece occurs. The number of blows required to cause this 

 failure represents the toughness. 



The cementing value, or binding power of a road material, is the property 

 possessed by a rock dust to act as a cement on the coarser fragments comprising 

 crushed stone or gravel roads. This property is a very important one, and is 

 determined approximately as follows: 



One kg. of the rock to be tested is broken sufficiently small to pass through 

 a 6 mm. but not a 1 mm. screen. It is then moistened with a sufficient amount 

 of water and placed in an iron ball mill containing two chilled iron balls weigh- 

 ing 25 pounds each and revolved at the rate of 2,000 revolutions per hour for 

 two hours and a half, or until all the material has been reduced to a thick dough, 

 the particles of which are not above 0.5 mm. in diameter. About 25 grams of 

 this dough is then placed in a cylindrical metal die, 25 mm. in diameter, and by 

 means of a specially designed hydraulic press, known as a briquette machine, is 

 subjected to momentary pressure of 100 kg. per square centimeter. Five of the 

 resultant briquettes, measuring exactly 25 mm. in height, are taken out and 

 allowed to dry for 12 hours in air and 12 hours in a hot oven at 100° C. After 

 cooling in a desiccator they are tested by impact in a machine especially con- 

 structed for the purpose. This machine is somewhat similar to that used in 

 determining the hardness, and the blow is about the same, excepting that it is 

 given by a 1 kg. hammer and the distance of drop does not exceed 10 cm. 



The standard fall of the hammer for a test is 1 cm. and the average number 

 of blows required to destroy the bond of cementation in the five briquettes 

 determines the cementing value. 



The specific gravity, is the weight of the material compared with that of an 

 equal volume of water, and is obtained by dividing the weight in air of a rock 

 fragment by the difference of its weight in air and water. Given the specific 

 gravity, the weight per cubic foot of a rock is found by multiplying this value 

 by 62.5 pounds* the weight of a cubic foot of water. 



The examination of a rock for structure, its mineral components and 

 the degree to which it has become weathered, is carried out by preparing 

 a thin section of such thickness as to be transparent under the micro- 

 scope. Its characteristics are then readily determined by the methods 

 employed by the petrographer for this purpose. The appearance of 

 such a section, made from a trap rock of a kind used in the construction 

 of a broken-stone road, is shown in an accompanying illustration, taken 

 from the Bulletin of the Office of Public Eoads, which has been re- 

 ferred to. 



The application of the methods which have been described to the 

 study of rocks for the purpose of determining their suitability and 

 relative merit for road construction was the first contribution to and 

 application of scientific methods to the subject. Before this road con- 



