[9] 



DIEECTIONS FOR COLLECTING EOCKS MEERILl 



I 



Fig. 10.— Balsam bottle. 



is referred to any authoritative work on tlie subject of liglit, and to 

 Prof. J. P. Idding's translation of Professor Eosenbuscli's work on 

 optical mineralogy. ^ 



But to return to the subject of making 



sections. A thin chip of the size of a 



nickel 5-cent piece, or at most not over an 



inch in diameter and as thin as j)ossible, is 



broken from that portion of the rock selected 



for study. By means of emery and water 



on a smooth cast-iron plate, one side is then 



ground until all inequalities are obliterated. 

 The final grinding 

 must be done with 

 emery so fine as 

 to leave no per- 

 ceptible scratches. 

 This chip is then 

 cemented, smooth 

 side down, against 



a small piece of ordinary double- thick 

 window glass, by means of Canada 

 balsam, such as may be procured at 

 almost any drug store, and which, in 

 its liquid form, is best kept in a capped 

 bottle as shown in fig. 10.^ The glass 

 serves merely as a holder and may be 

 of any convenient size. Eectangular 

 pieces one by two inches have been 

 found most convenient in this labora- 

 tory. In cementing the chip a few 

 drops of the liquid balsam are placed 

 upon the center of the glass slip, which 

 is then gently heated until such quan- 

 tity of the volatile constituents pass 

 olf that on cooling the residue is hard 

 but not brittle. Experience is here 

 the only guide; if heated too long it 

 will become brittle and break away; 

 if not long enough it is soft and sticky. 

 Hard enough to be impressed slightly 

 by the thumb nail without sticking or 

 cracking is as definite as can be well 

 stated. In case a large number of 

 sections are to be prepared it is well 



iMicroscopic Physiography of Eock-Making Minerals. Wiley & Sons, New York. 

 2 Some workers profess to prefer a mixture of Tsnetian turpentine and white shel- 

 lac boiled down to proper consistency. 



Fig. 11. Fig. 12. 



Mounting needle and forceps. 



