r, and of various thicknesses, will
also be required for special purposes.

Thermometer and "barometer" tubing is occasionally required, the
latter, by the way, making particularly bad barometers. The
thermometer tubing should be of all sizes of bore, from the finest
obtainable up to that which has a bore of about one-sixteenth of an
inch. Glass rods varying from about one-twentieth of an inch in
diameter up to, say, half an inch will be required, also two or three
sticks of white enamel glass for making joints.

To facilitate choice, there is appended a diagram of sizes from the
catalogue of a reliable German firm, Messrs. Desaga of Heidelberg, and
the experimenter will be able to see at a glance what sizes of glass
to order. It is a good plan to stock the largest and smallest size of
each material as well as the most useful working sizes.

Fig. 1.

Sec. 11. Testing Glass.

"Reject glass which has lumps or knots, is obviously conical, or has
long drawn-out bubbles running through the substance." If a scratch be
made on the surface of a glass tube, and one end of the scratch be
touched by a very fine point of fused glass, say not more than
one-sixteenth inch in diameter, the tube, however large it is (within
reason), ought to crack in the direction of the scratch. If a big
crack forms and does not run straight, but tends to turn
longitudinally, it is a sign that the glass is ill annealed, and
nothing can be done with it. If such glass be hit upon in the course
of blow-pipe work, it is inadvisable to waste time upon it; the best
plan is to reject it at once, and save it for some experiment where it
will not have to be heated.

The shortest way of selecting glass is to go to a good firm, and let
it be understood that if the glass proves to be badly annealed it will
be returned. Though it was stated above that the glass should not be
distinctly conical, of course allowance must be made for the length of
the pieces, and, on the other hand, a few highly conical tubes will be
of immense service in special cases, and a small supply of such should
be included.

The glass, as it is obtained, should be placed in a rack, and covered
by a cloth to reduce the quantity of dust finding its way into the
tubes. It has been stated by Professor Ostwald that tubes when reared
up on end tend to bend permanently. I have not noticed this with lead
glass well supported. Each different supply should be kept by itself
and ca