MICROSCOPIC SECTIONS OF ROCKS 1067 



this purpose they should be from about y^th 'to r -^ Tt th of an inch 

 thick. 



A chip about an inch square is struck or cut off the specimen to 

 be studied. One surface of this is then ground down on a flat cast- 

 iron plate with emery and Avater. This grinding may be done either by 

 hand or by means of a machine specially constructed for this purpose 

 (Chap. VII). 1 The former method will be described here. When 

 a smooth surface is at last obtained the specimen is well washed with 

 water and then polished upon a slab of plate glass with the finest 

 flour emery and water. When all inequalities are thus removed the 

 fragment is again well cleansed from all adhering emery. 



The next process is to cement it with Canada balsam upon a slab 

 of glass about two inches square and about an eighth of an inch in 

 thickness. The Canada balsam is first heated over a spirit lamp in 

 an iron spoon, care being taken not to allow it to burn. This is the 

 most difficult part of the whole process, and only experience can teach 

 how long the balsam must be heated in order to possess, on cooling, 

 the necessary hardness. If it be heated too long it will crack upon 

 cooling. The right point appears to be that in which large air-bubbles 

 force themselves through the viscous mass. 



A small quantity of the warm balsam is poured upon the slab of 

 glass, and the smooth surface of the rock-fragment, being pressed into 

 the balsam, is held down upon the glass till the balsam hardens. The 

 slab is then examined from its under side to see that no air-bubbles 

 have been included between the glass and the stone. Should they be 

 present in any quantity, the whole process must be repeated. When 

 the balsam has quite hardened, the other side of the fragment is 

 ground down with coarse emery and water on the iron plate. Upon 

 the section commencing to become transparent, the grinding with the 

 coarse emery must cease. The stone is then thoroughly cleansed 

 with water, and the final grinding is conducted upon the plate-glass 

 slab with flour emery and water. 



The slide is then placed under a stream of water in order to 

 remove all traces of the emery powder from the minute pores of the 

 rock. This is now the time to employ chemical tests to the com- 

 ponent minerals, if such a course be deemed advisable. If the rock 

 is of a fragile nature, it is well to mount the section as it is ; but in 

 most cases it is possible by delicate manipulation- to remove it to a 

 mounting more suited to optical work. This transference is effected 



1 F. G. Cuttell (61 Camden Eoad, N.W.), T. Eiley (18 Burnfoot Avenue, 

 Fulham, S.W.), and J. Ehodes, Museum of Geology, Jermyn Street, S.W., prepare 

 good sections ; and the principal petrological opticians can generally recommend 

 efficient operators. Voigt and Hochgesang (Gb'ttingen, Rothe Str. 13) and R. Fuess 

 (Berlin, S.W., 108 Alte Jacob Str.) do also most excellent work. German craftsmen 

 are more skilful in overcoming difficulties (e.g. with soft rocks) than English, and 

 can make thinner slices. Hence, it is better to send specimens to Germany when 

 thinness is desired ; but when the size of the slice is important, to have the work done 

 in England. In a very thin slice the colour phenomena are less conspicuous, so 

 that reduction in thickness beyond a certain limit is not all gain ; but in rocks of 

 an opaque character, or in the study of very minute structures, it is hardly possible 

 to err on the side of thinness, and slices ' made in Germany ' are much the better. 

 If a student is purchasing ready made specimens from a dealer, he will find the 

 following rough test useful. Look through the slice at a window with a clear sky 

 beyond ; it is too thick when the bar cannot be distinctly seen. 



