124 
BULLETIN OF THE LABORATORIES 
thin perfect glass for mounting the sections, the size preferred being 
45 by 25 mm. and is furnished by any dealer in microscopes; (3) a 
corresponding number of covers of thin glass, square in shape and 
large enough to extend nearly across the slip; (4) ajar of balsam nearly 
solid or of the consistency of honey and lumps of solid balsam, (the 
balsam thould be in a wide-mouthed bottle covered with a protecting 
shield or glass stopper, through which extends a dipping rod); (5) sev- 
eral grades of emery powder, it being convenient to hd:ve numbers o, 
and I, and emery flour, as well as a small quantity of emery slime. ^ 
This last is not kept by ordinary dealers and is made by decanting off 
the finest impalpable powder during the grinding of other grades. It 
may be secured of Julien, of New York, or through Bausch & Lomb, 
of Rochester; (6) an alcohol or gas lamp; (7) a heating table or tri- 
pod with brass plate, for heating the slides ; (8) a number of spring 
clothes pins, with the lips filed flat, to hold the cover glasses while the 
balsam drys ; (9) a bottle of solution for cleansing the glasses, which 
is prepared from sulphuric acid and chromate of potash ; (10) a contri- 
vance of some sort for grinding or sectioning the rock. The simplest 
way is, after breaking as thin a fragment as possible from an unweathered 
sample, to grind one side of the fragment upon a smooth iron surface with 
coarse emery and water until a plane surface is secured as large as a quar- 
ter or larger. The surface is then ground with emery flour upon a large, 
smooth plate of glass, after which a polish is secured by long rubbing 
on a second plate with emery slime. The surface thus prepared is 
glued to one of the squares of plate glass with balsam. The best 
results are secured by using balsam which is quite hard but not yet 
brittle ; it should yield to the nail with difficulty, but should not shiver 
into fragments. A small piece is placed on the cleaned surface of the 
square and set on the brass plate of the tripod over the flame of the 
lamp. The chip of rock is also heated at the same time. When 
the balsam has become thoroughly fluid, but before bubbles appear, 
the section is pressed firmly down upon the balsam and as much of it 
pressed out as possible. Care should be taken that no bubbles or im- 
purities find their way between the section and the glass. A weight 
or spring may, in some cases be necessary to prevent the slightest ele- 
vation of one side of the chip from the glass square. The balsam 
will set very quickly and, if the heating has been slow enough, will 
have become tough — it should no longer yield to the nail. The older 
lithologists advise the use of soft balsam, which is heated until it ac- 
