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cement as betore. When the chip has been reduced until it is 
tolerably thin; until, for example, light appears through it when 
held between the eye and the window, we may, as before, wash 
it clear of the coarse emery and continue the reduction of it on the 
glass plate with fine emery. Crystalline rocks, such as granite, 
gneiss, diorite, dolerite, and modern lavas, can be thus reduced to the 
required thinness on the glass plate. Softer rocks may require 
gentle treatment with the Water of Ayr stone. 
The last parts of the process are the most delicate of all. We 
desire to make the section as thin as possible, and for that purpose — 
continue rubbing until after one final attempt we may perhaps find to 
our dismay that great part of the slice has disappeared. ‘The utmost 
caution should be used. ‘The slide should be kept as flat as possible, 
and looked at frequently, that the first indications of disruption may 
be detected. The thinness desirable or attainable depends in great 
measure upon the nature of the rock. Transparent minerals need 
not be so much reduced as more opaque ones. Some minerals, indeed, 
remain absolutely opaque to the last, like pyrite, magnetite, and 
ilmenite. 
The slide is now ready for the microscope. It ought always to be 
examined with that instrument at this stage. We can thus see 
whether it is thin enough, and if any chemical tests are required they 
can readily be applied to the exposed surface of the slice. If the 
rock has proved to be very brittle, and we have only succeeded in 
procuring a thin slice after much labour and several failures, nothing 
further should be done with the preparation, unless to cover it with 
glass, as will be immediately explained, which not only protects it, 
but adds to its transparency. But where the slice is not so fragile, 
and will bear removal from its original rough scratched piece of glass, 
it should be transferred to one of the glass-slides (No. 10). For this — 
purpose the preparation is once more placed on the warm iron plate, 
and close alongside of it is put one of the pieces of glass which has 
been carefully cleaned, and on the middle of which a little Canada 
balsam has been dropped. ‘The heat gradually loosens the cohesion 
of the slice, which is then very gently pushed with the needle or 
knife along to the contiguous clean slip of glass. Considerable - 
practice is needed in this part of the work, as the slice, being so 
thin, is apt to go to pieces in being transferred. A gentle inclination 
of the warm plate, so that a tendency may be given to the slice to 
slip downwards of itself on to the clean glass, may be advantageously - 
given. We must never attempt to lift the slice. All shifting of its 
position should be performed with the point of the needle or other 
sharp instrument. If it goes to pieces we may yet be able to pilot 
the fragments to their resting-place on the balsam of the new glass, 
and the resulting slide may be sufficient for the required purpose. 
When the slice has been safely conducted to the centre of the 
glass slip, we put a little Canada balsam over it, and warm it 
as before. Then taking one of the thin cover-glasses with the 

