METHODS OF TESTING SLATE. 47 
Strength.— Merrhnan's apparatus for testing the strength of slate is described in 
his paper :« 
The pieces were supported in a horizontal position upon wooden knife-edges 22 inches apart, and 
these loads Avere placed upon another knife-edge halfway between the supports. This load was 
applied by means of sand running out of an orifice in a box at the uniform rate of 70 pounds per 
minute, and by the help of an electric attachment the flow was stopped at the instant of rupture. 
He computes the modulus of rupture by the use of this formula, S=Ai\, in which 
w is the load in pounds which causes the rupture, 1 the distance between the sup- 
ports in inches, b the breadth and d the thickness of the plate in inches. S in tin- 
number of pounds per square inch. According to these tests the modulus of rupture 
in the best slates should range from 7,000 to 10,000 pounds. 
J. F. Williams & made tests of compression and elastic limit on purple, red, and 
green slates from Rutland and Washington counties. His result- give a modulus of 
rupture ranging from 7,310 to 10,800 pounds per square inch. Campbell & Donald ' 
give 20,000 pounds as the crushing weight for 1 cubic inch of slate. Wilkinson, in 
his Practical Geology of Ireland, gives 30,730 pounds as the crushing weight of the 
Killaloe slates. d Watrin « gives the maximum crushing weight of some French shit. >s 
as 2,000 kilograms per square centimeter, but 1,700 as the average. 
Merriman has also devised an impact test for determining the strength of slates. 
He lets a wooden ball weighing 15.7 ounces fall 9 inches upon a piece of slate meas- 
uring 6 by 7| inches by from 0.20 to 0.28 inches thick, and repeats the blows until 
the slate breaks. The foot pounds of work per pound of slate are then calculated 
from the weight and thickness of the slate and the number of blows. The resulting 
figures range from 3.50 to 126.66. 
Toughness or elasticity. — Merriman finds the ultimate deflection in certain Pennsyl- 
vania slates, when placed on supports 22 inches apart, to range from 0.270 to 0.313 
inch. Certain blue-black slates in Eldorado County, Cal., when split seven to 
the inch and 18 inches square and fastened solidly at the two ends are said to bend 
3 inches in the center without any sign of fracture./ J. F. Williams 9 tested beams 
of slate from Rutland and Washington counties, 1 inch square and 10 inches long, 
with supports 6 inches apart. Bending without breaking was effected by from 770 
to 1,200 pounds, and when the supports were placed 3 inches apart by from 1.710 to 
2,400 pounds. The great elasticity of the slates of western Vermont and of North- 
ampton and Lehigh counties, Pa., is apparent to any one visiting the shanties where 
the splitting is done. 
Density or specific gravity. — This is determined in the usual way, by weighing a 
piece of the slate in and out of water and dividing its weight out by the difference 
between its weight in and out. All air should first be removed by boiling the piece 
of slate in distilled water. The specific gravity will be considerably affected by the 
amount of magnetite or pyrite. 
Porosity.— This is best determined by drying, then weighing, then immersing for 
twenty-four hours and weighing again, in order to ascertain the percentage of water 
absorbed. Merriman takes a piece 3 by 4 inches, with rough «-dges, dries it in an 
oven at 135° F. for twenty-four hours, cools to the normal temperature of room, 
weighs, and immerses it for twenty-four hours, and weighs again. His tests of IVnn- 
«SeeBibliog.. p. 144. An extended abstract of Merriman's paper is givrn bj Win. c. Daj in part6, 
continued, of Nineteenth Annual Report (pp. 257-263), and on p. 122 of this bulletin the results of his 
recent tests of 33 specimens of slate are given. 
&See Bibliog., p. 145. 
cEncycl. Brit., ninth ed., 1887. 
d Quoted by Hull; see Bibliography, p. 144. 
* Les ardosieres des Ardennes, pp. 192, 193. . . T _ . . _ . .,, . onA 
/California State Mining Bureau, twelfth report of State Mineralogist J. J. Crawford, Sept. 15, 1894, 
p. 400. 
0See Bibliography, p. 145. 
