the
socket; the socket at this stage being shown by _f_. The "blank" for the
socket is now completed, but it must be heated to dull redness in order
to relieve strain and be placed in an annealing oven, where it should
be annealed for some hours.

The "blank" for the plug offers no special difficulty; it is made by
heating a glass rod and compressing it axially until a mass having the
form shown by _g_, Fig. 14, is produced; the end of this is heated
intensely and brought in contact with the rather less heated side of a
glass tube which has been drawn to the shape desired for the handle;
when contact is made a slight air pressure is maintained in the glass
tube, thus producing a hollow join. The ends of the tube are sealed and
the bottom of the plug is drawn off, thus giving the finished "blank" as
shown by _h_. This blank is now held in a pair of asbestos-covered
tongs, heated to dull redness all over, and transferred to the annealing
oven.

When cold, the socket is ground out by the second method given under
"Grinding Stoppers"; that is to say, by means of type-metal or copper
cone, and the plug is ground to fit in a corresponding mould. When the
fit is almost perfect, the transverse hole is drilled in the plug, and
the final finishing is made with fine abrasive powder. Great care must
be taken in the final grinding that there is no accumulation of
abrasive material in the transverse hole of the plug; if this is allowed
to occur there will be a ring ground out of the socket where the holes
move, and the tightness of the finished stopcock will be lost.

_Marking Glass._--As a preliminary to a consideration of the methods of
graduating and calibrating glass apparatus, it is convenient to consider
the various methods which are available for marking glass. Among these
are, the writing diamond, the carborundum or abrasive pencil, the
cutting-wheel, and etching by means of hydrofluoric acid. Each produces
a different class of marking and each is worthy of independent
consideration.

_The Writing Diamond._--This is the name given to a small irregular
fragment of "bort" which is usually mounted in a thin brass rod. Such a
diamond, if properly selected, has none of the characteristics of a
cutting diamond; although one occasionally finds so-called "writing
diamonds" which will produce a definite cut. These should be rejected.

The writing diamond is used in much the same way as a pencil, but is
held more perpendicularly