ot very well known outside geological circles is
the manipulation of the diamond-cutting wheel, and as this is often of
great use in the physical laboratory, the following notes may not be
out of place. I first became acquainted with the art in connection
with the necessity which arose for me to make galvanometer mirrors out
of fused quartz, and it was then that I discovered with surprise how
difficult it is to obtain information on the point. I desire to
express my indebtedness to my colleagues, Professor David and Mr.
Smeeth, for the instruction they have given me. In what follows I
propose to describe their practice rather than my own, which has been
of a makeshift description. I will therefore select the process of
cutting a slice of rock for microscopical investigation.

Sec. 76. Arming a Wheel.

Fig. 63.

A convenient wheel is made out of tin-plate, i.e. mild steel sheet,
about one-thirtieth of an inch thick and seven inches in diameter.
This wheel must be quite flat and true, as well as round; too much
pains cannot be taken in securing these qualities. After the wheel is
mounted, it is better to turn it quite true by means of a
watch-maker's "graver" or other suitable tool. The general design of
a rock-cutting machine will be clear from the illustration (Fig. 63).

The wheel being set up correctly, the next step is to arm it with
diamond dust. For this purpose it is before all things necessary that
real diamond dust should be obtained. The best plan is to procure a
bit of "bort" which has been used in a diamond drill, and whose
properties have therefore been tested to some extent. This is ground
in a diamond mortar--or rather hammered in one--and passed through a
sieve having at least 80 threads to the inch. The dust may be
conveniently kept in oil.

To arm the wheel, a little dust and oil is taken on the finger, and
laid on round the periphery of the wheel. A bit of flint or agate is
then held firmly against the edge of the wheel and the latter is
rotated two or three times by hand. The rotation must be quite
slow--say one turn in half a minute--and the flint must be held firmly
and steadily against the wheel. Some operators prefer to hammer the
diamond dust into the wheel with a lump of flint, or agate, but there
is a risk of deforming the wheel in the process. When a new wheel is
set up, it may be necessary to repeat the above process once every
half hour or so till the cutting is satis