MICEOSCOPIC STRXJCTUBB 39 



Tubbing it in water and emery on a smooth, cast-iron plate. 

 Toward the close of the process fine flour of emery is used, 

 as the final surface must be very smooth and free from scratches. 

 This chip is then cemented smooth side down on a piece of 

 ordinary double-thick window glass, a convenient size being 

 about 2X1 inches, the cementing material being Canada balsam 

 which has been evaporated to the extent that, when cold, it is 

 sufficiently hard to hold firmly, is not at all sticky, but yet is not 

 so hard as to be brittle. The exact degree can only be learned 

 by experience ; a hardness such as to be barely indented by the 

 thumb nail will be found about right. This operation of ce- 

 menting is best done by means of a thin iron plate laid hori- 

 zontally on a support and heated not too hot by a lamp beneath. 

 The glass with the balsam upon it is heated to the right tem- 

 perature, the balsam being fluid and free from bubbles. The 

 rock chip, heated sufficiently to expel all moisture, is then pressed 

 firmly into the balsam, in such a way as to exclude air bubbles, 

 and brought within as close contact with the glass as possible. 

 It is then removed from the iron plate and allowed to cool, 

 when the grinding process is resumed, the glass plate serving 

 merely as support for the film of stone and something for the 

 fingers to hold by. Being transparent, the worker can see just 

 how the grinding is progressing without continually stopping to 

 examine. When sufficiently thin, — usually from ^Vo- ^^ -^ 

 of an inch, — the film is remounted as follows: While on the 

 thick glass on which it was ground, it is thoroughly washed 

 with a brush — an ordinary tooth-brush serves well — to get 

 rid of all particles of emery and other dirt that may adhere. It 

 is then washed in alcohol to get rid of the old hard balsam, which 

 is usually quite dirty from mud produced in grinding. Fresh 

 mounting slips and clean cover glasses being ready, the first is 

 laid upon the warm iron plate with a couple of drops of balsam 

 in the centre, and allowed to heat until it begins to smoke. 

 Care must here be exercised, as, if heated too much, the balsam 

 becomes hard and brittle, and if too little, the mount is sticky 

 from the balsam which constantly oozes from under the cover. 

 The thick glass, with its film of stone still adhering, is likewise 

 laid upon the warm iron plate, and a drop of fresh balsam placed 

 upon the film. This is then gently heated, and the cover-glass, 

 first warmed, gently laid upon it — one edge placed in position 

 and lowered gradually in such a manner as to force out any acci- 



