" the tube will break at once. As a supply
of electrical current for lighting will, in the near future, be as
much a matter of course for laboratory purposes as a gas supply, I add
the following note. To heat a tube round a scratch, nothing--not
even the oxygas blow-pipe--is so good as a bit of platinum or iron
wire electrically heated. If the crack does not start by considerable
heating of the glass, stop the current, unwind the wire, and touch the
glass on the crack either with a bit of cold copper wire or a wet
match stem. I prefer the copper wire, for in my experience the water
will occasionally produce an explosion of cracks. On the other hand,
the cold wire frequently fails to start a crack.

Judging from the appearance of thick tubes as supplied by the dealers,
the factory method of cutting off appears to be to grind a nick almost
through the tube, and right round; and for really thick glass this is
the safest but slowest way; a thin emery wheel kept wet will do this
perfectly. Suitable wheels may be purchased from the "Norton" Emery
Wheel Co. of Bedford, Mass, U.S.A.--in England through Messrs.
Churchill and Co. of London, importers.

Sec. 31. To blow a Bulb at the End of a Tube.

I must admit at once that this is a difficult operation--at all
events, if a large bulb is required. However, all there is to be said
can be said in few words. In general, when a bulb is required at the
end of a tube it will be necessary to thicken up the glass. A
professional glass-worker will generally accomplish this by "jumping
up" the tube, i.e. by heating it where the bulb is required, and
compressing it little by little until a sufficient amount of glass is
collected. The amateur will probably find that he gets a very
irregular mass in this way, and will be tempted to begin by welding on
a short bit of wide and thick tubing preparatory to blowing out the
bulb.

However, supposing that enough glass is assembled by-either of these
methods, and that it is quite uniform in thickness, let the thickened
part be heated along a circle till it becomes moderately soft, and let
it then be expanded about one-fifth, say by gently blowing. It is
perhaps more important to keep turning the glass during bulb-blowing
than in any other operation, and this both when the glass is in the
flame and while the bulb is being blown. It is also very important to
avoid draughts. In general, a bulb is best blown with the tube in a
near