1878. ] Microscopical Examination of Rocks. - 221 
arranged in such a manner that no one operation interferes with 
another. 
The grinding and slicing is performed by the aid of emery, 
which is constantly applied to the plates and slicer with a brush 
held in the left hand. The plates are kept wet by a small current 
of water. It is important to use emery of different but uniform 
degrees of fineness as the work progresses. A mistake usually 
made by beginners is to choose a too coarse emery, in order to 
hasten the work of grinding, but they soon discover that all their 
sections break and wear off long before being half finished. It is 
advisable to use for the coarse grinding on plate A an emery 
which can be bought in any drug store as emery No. 90 (A). 
‘Plate B is charged with flour of emery (B); it serves also to pre- 
pare one or two sorts of finer emery from it, by treating a certain 
quantity of the flour with water in a vessel which may be about 
three feet high and six inches in diameter. In stirring the water, 
the emery becomes suspended in it; after a lapse of fifteen min- 
utes the water with emery which has not settled down during 
this time is decanted in a similar vessel and left there for twenty 
minutes more, when the water may be drained off. A sediment 
of fine emery will thus form in the latter vessel; and after having 
repeated the process for some time the emery sediment (emery 
C) is taken from the vessel, dried and bottled. Its degree of fine- 
ness is expressed by the number of minutes, which is in this case 
fifteen minutes. 
Canada balsam serves as a cementing medium, it also is used 
‘to increase the transparency of various sections. It should be 
entirely clear and not of yellowish tint: Usually a solution of 
Canada balsam in turpentine is employed to mount such rocks as 
= will not undergo any change in heating them gently over a warm 
plate of iron. For rocks which cannot be exposed to any heat, 
a solution of pure Canada balsam in chloroform is used. Both 
.solutions of Canada balsam should be well bottled to prevent 
evaporation. 
= Object Glasses —The slides of glass which are intended to bear 
the section should be entirely clear, free of any color, air bubbles 
_or any other enclosures, as those which are used in other branches 
of microscopy. Their size should be uniform. Prof. Zirkel, of 
Leipzig, the distinguished lithologist, has proposed and introduced 
the howe size, 45 mm. by 25 mm., which is preferable to the 
