46 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES. 
surface. The greased area reached by the heat will, in cooling, leave an oval out- 
line, the long axis of which will show the direction of the grain, the heat having 
traveled more rapidly within the slate in the direction of the grain than in any other. 
He also made a disk of slate 5 inches in diameter of ordinary thickness, with a 
central perforation. This disk was fastened by the extremities of the diameter 
parallel to the grain and afterwards by that at right angles to the grain, and was 
made to vibrate by tapping the side of the perforation. The sound produced when 
the disk was fastened by the diameter at right angles to the grain was louder than 
when fastened by that parallel to it. In other words, the direction of the grain was 
that in which elasticity and vibration were greatest, 
Character of cleavage surface. — The cleavage surface should be examined with an 
ordinary magnifying glass. A superior slate should scale along the cleavage surface 
into very thin chips with translucent edges. If the grain is pronounced it will appear 
in fine transverse lines. If false cleavage, which is fatal, be present, it can usually 
be detected on the cleavage surface. Ribbons, which are sometimes lines of weak- 
ness, should be noted. There is great difference in the smoothness of the surface in 
slates of different regions. Ordinarily the constituent minerals ought not to be 
visible. Minute lenses or crystals an- nut necessarily detrimental, but they retain 
dust and thus afford a foothold for mosses and other cryptogams, which gather 
moisture and thus aid the decomposition of the slate. 
Presenct of lime. — This can be determined by the application of cold dilute hydro- 
chloric acid to the edges of a Ereshly-quarried slate. Rapid effervescence implies 
presence of carbonate of lime; slow, that of a lesser quantity of it or of dolomite — 
carbonate of lime and magnesia. Reverdin and de la Harpe call attention to the 
fact that good slate may have a high percentage of calcium carbonate, while others 
free from it may be poor. But it is nevertheless true that slates containing such a 
percentage of lime must, other things being equal, in time be acted upon more' 
powerfully by the acids of the atmosphere than those without any or with an infini- 
tesimal one. 
Color and discoloration. — The color of the freshly-quarried slate should be noted 
and com pa led with that of pieces exposed for several years to the weather, either 
on a roof or on the quarry dumps, or with that at the top of the quarry close to the 
gravel, although this last comparison may not always be perfectly conclusive. The 
value of slates is somewhat affected by the extent of their discoloration. 
Presence of clay. — This should be tested by breathing upon a fresh and clean pied 
of slate and observing whether there is any argillaceous odor. The very best slat( 
will not emit any such odor. 
Presence of marcasite. — A slate containing lenses or crystals of a pale-yellowisl 
metallic mineral which, on exposure, decomposes, forming a yellowish white fill! 
and rusty spots, is poor; ft but pyrite in small quantity is not necessarily detrimental 
for it does not decompose readily. It is present in nearly all slates, and particular!; 
in black slate. 
Present of magnetite. — Since slate has come to be used so largely in electric appli 
ances the determination of the relative abundance of this mineral is of practics 
importance. A cubic inch of the slate should be weighed, broken up, and reduce 
to a tine powder in an agate mortar or else between sheets of zinc or copper, an 
then spread on a flat surface and thoroughly traversed by a strong magnet and tl 
extracted magnetite weighed. The percentage, by weight, per cubic inch of slate ca 
thus be determined. 
a See Bibliography, p. 145. 
*>See Alexis A. Julien. On the variation of decomposition in the iron pvrito: its cause, and i 
relation to density: Annals X. Y. Acad. Sci., vol. 3. pp. 365-40:'.: vol. 4. pp. 125-221 an. I Pis. VIII, I 
1886, 1887. Also, A. P. Brown. On marcasite and pyrite; a comparative study of the chemical behayj 
of pyrite and mareasite: Proc. Am. Phil. Soc, vol". 33, pp. 225-243, 1394. Also, H. N. Stokes. On pyii 
and marcasite: Bull. U. S. Geol. Survev No. 186, 1901. 
