48 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES. 
sylvania slates showed from 0.099 to 0.303 per cent of absorption. Porosity is some- 
times roughly indicated by immersing a roofing slate edgewise one-half in water and 
observing how far the water ascends by capillary attraction. In good slates it ought 
to rise but very little. 
Reverdin and de la Harpe a state that slates are liable to deterioration from the 
chemical action of gases arising from woodwork beneath the slate, as well as from 
the action of the atmosphere, and that they are also liable to an increase of porosity 
by the physical action of changes of temperature and by the unequal conductivity of 
heat in the direction of cleavage and of grain. They state that the porosity in a fresh 
slate should be below 0.1 per cent, and after treatment less than 0.2 per cent. Their 
somewhat elaborate method is this: For determining porosity as produced by acids 
the slate is treated with 10 per cent cold acetic acid and the flask is made vacuous 
from time to time. The piece is then washed, dried, weighed, and immersed in 
diphenylamine in a thick-walled tube 12 by 3J centimeters. The tube is exhausted, 
heated two hours in oil bath at 170° C, air pressure is restored, and heating con- 
tinued for four to five hours at 150° C, after which the test pieces are removed, the 
diphenylamine wiped off with ether, and the increase in weight taken. 
For determining porosity as produced by changes of temperature, the slate is heated 
in a wrought-iron tube for half an hour to 300° C, and the tube is then suddenly 
cooled by a stream of water for half an hour. This process is repeated twenty-four 
times, and the slate is then impregnated with diphenylamine and the procedure is as 
in previous test. 
Fresenius is accredited with a method of testing the effect of heat and cold on slate 
by saturating it with water and putting it for twenty-four hours in a freezing mixture 
and heating another from 250° to 350° for five or six hours and then immersing it in 
water. The porosity, strength, and elasticity of the pieces so treated should then 
be tested. Bottinger points out that the greater the porosity of a slate the more 
damaging is the action of frost likely to be. The effect of frost on the microscopic 
structure has already been referred to (p. 24). 
Hardness, or abrasion. — Merriman has also devised a method of testing the relative 
hardness of slates by subjecting a piece of determined weight to the action of a grind- 
stone revolved 50 times, the slate being held against it by a lever with a constant 
pressure of 10 pounds. The loss of weight in the process was then determined. As 
the softness depends largely upon the amount of quartz and mica a microscopic 
analysis would also throw light upon this quality. 
Corrodibility. — An important quality in roofing slates is their resistance to the acids 
of the atmosphere, particularly in cities, where gases increase its destructive power. 
Fresenius in 1868 suggested testing the weathering qualities of a slate by immersing 
it for three days in dilute sulphurous acid in a closed vessel. At the end of that time 
poor slates are softened or broken up into thin laminae or easily fractured, while good 
ones preserve both their density and hardness. 
Merriman for the same purpose prepared a solution consisting of 98 parts of water, 
1 part of hydrochloric acid, and 1 part of sulphuric acid. Pieces of slate 3 b} r 4 inches 
were carefully weighed, then immersed in the solution for one hundred and twenty 
hours, then dried for forty hours, weighed, the solution strengthened, and specimen 
reimmersed for another one hundred and twenty hours, and weighed again. This 
was repeated once more. The losses in weight ranged from to 2.76 per cent. 
Microscopic ancdysis. — One of the most satisfactory tests of slate is the examination 
of a thin section of it under the microscope. A cubic inch thus tested will suffice to 
show the character of the cleavage, the presence of false cleavage, if any, the prob- 
able durability, or indurability, of the color, as well as the presence of any mineral 
constituents likely to determine its other qualities. The specimen should be carefully 
a op. cit. 
