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FERE Pa Ee Pore ee E aa SR 
1878.] Microscopical Examination of Rocks. 223 
Mounting Process—A slab of iron two and a half feet long, 
four inches broad and half an inch thick is laid horizontally upon 
a tripod (of iron) and heated with a Bunsen burner. The glass 
slide on which the rock fragment is to be mounted is well cleaned 
and laid upon a part of the slab which is of a constant and mod- 
erate heat; after being warmed a few drops of Canada balsam are 
placed upon the glass and left with it on the slab till the balsam 
by evaporation has assumed a tenacious condition, in which its 
films will not adhere much to the fingers and yet be pliable 
enough to be bent readily without breaking. The warmed min- 
eral slice is now laid upon the Canada balsam with its ground 
face down, and allowed to remain on the slab till all air bubbles 
have disappeared below the slide in the balsam. It is then 
gently pressed on the glass and then taken off and allowed to 
cool. It is of great importance to use Canada balsam in its 
above described condition, as the rock section during the process 
of grinding thin unavoidably breaks to pieces in case the balsam 
proves to be too soft, and if on the contrary it has been too long 
exposed on the slab it will become brittle and the section will, 
during the grinding, be liable to break off. The loosened min- 
eral slice must in such a case be laid into a dish with turpentine 
to dissolve the balsam, and after being well cleaned is mounted 
again as described. There is no considerable difference in the 
process of mounting with balsam in solution with chloroform or 
with turpentine. The balsam in chloroform is allowed to harden 
in a warm room: it usually needs one or two days, while the solution 
in turpentine by application of heat hardens in from five to ten 
minutes. In the latter case care should be taken not to heat the 
balsam too quickly, as it turns yellowish on account of a partial 
carbonizing. 
Grinding Operation.—After having used all necessary precau- 
tion in cementing the rock fragments to the glass, the grinding 
Manipulation now begins on the reverse side of the section. It 
is ground at first on plate A (emery A), then on plate B (emery 
B) till the section commences to be translucent, or till it is so 
thin that it is not advisable to continue the process on the rotating 
Wheel. It is, of course, only a matter of experience to know how 
long to use the various wheels, as much depends on the consis- 
_ tency of the rock material. It is advisable to discontinue the use 
of plate B before the slice is too much reduced in thickness, so 
