462 SUMMARY OF CURRENT RESEARCHES RELATING TO 



machine, provided the operator possesses ordinary neatness of hand 

 and practice. A dexterous use of the hammer will break off from a 

 sharp edge of the rock a number of thin si^linters or chips about an 

 inch square. 



For the preparation of the thin slices for the Microscope, the 

 following simple apparatus is all that is absolutely needful, though if 

 a grinding machine is added it will save time and labour : — (1) A 

 cast-iron plate 9 in. square and ^ in. thick ; (2) two i:)ieces of plate- 

 glass 9 in. square ; (3) a Water-of-Ayr stone 6 in. x 2|- in. ; (4) 

 coarse emery ; (5) fine or flour emery ; (6) putty powder ; (7) Canada 

 balsam ; (8) a small forceps, and a common sewing needle in a wooden 

 handle ; (9) some oblong pieces of common flat window-glass 2 in. x 

 1 in.; (10) glass slides with ground edges; (11) thin cover-glasses; 

 (12) a small bottle of spirits of wine. The following are the direc- 

 tions given by Dr. Geikie for the subsequent processes : — 



" The first j^rocess consists in rubbing down and polishing one side 

 of the chip or slice (if this has not already been done in cutting off a 

 slice affixed to glass, as above mentioned). We place the chip upon 

 the wheel of the grinding-machine, or, failing that, upon the iron 

 plate, with a little coarse emery and water. If the chip is so shaped 

 that it can be conveniently pressed by the finger against the plate, 

 and kept there in regular horizontal movement, we may proceed at 

 once to rub it down. If, however, we find a difficulty, from its small 

 size or otherwise, in holding the chip, one side of it may be fastened 

 to the end of a bobbin or other convenient bit of wood by means of a 

 cement formed of three parts of rosin and one of beeswax, which is 

 easily softened by heating. A little practice will show that a slow, 

 equable motion with a certain steady pressure is most effectual in pro- 

 ducing the desired flatness of surface. When all the roughnesses 

 have been removed, which can be told after the chip has been dipped 

 in water so as to remove the mud and emery, we place the specimen 

 upon the square of plate-glass, and with flour-emery and water con- 

 tinue to rub it down until all the scratches caused by the coarse emery 

 have been removed, and a smooth polished surface has been produced. 

 Care should be taken to wash the chip entirely free of any grains of 

 coarse emery before the polishing on glass is begun. It is desirable 

 also to reserve the glass for polishing only. The emery gets finer 

 and finer the longer it is used, so that by remaining on the plate it 

 may be used many times in succession. Of course the glass itself is 

 worn down, but by using alternately every portion of its surface, and 

 on both sides, one plate may be made to last a considerable time. If 

 after drying and examining it carefully we find the surface of the chip 

 to be polished and free from scratches, we may advance to the next 

 process. But it will often happen that the surface is still finely 

 scratched. In this case we may place the chip upon the Water-of- 

 Ayr stone, and with a little water gently rub it to and fro. It should 

 be held quite flat. The Water-of-Ayr stone, too, should not be 

 allowed to get worn into a hollow, but should be kept quite flat, 

 otherwise we shall lose part of the chij). Some soft rocks, however, 

 will not take an unscratched surface even with the Water-of-Ayr 



