d tube is to be
introduced, and proceeding as in the case of the air-trap just
described.

Ozone generators of the form shown on next page (Fig. 19), afford an
interesting example of the insertion of smaller tubes into larger.

On account of the small space that may be left between the inner and
outer tubes of an ozone generator, and of the length of the inner tube,
its construction needs great care. I find the following mode of
procedure gives good results. Select the pieces of tube for this
instrument as free from curvature as possible. For the inner tube, a
tube 12 mm., or rather more, in external diameter, and of rather thin
glass, is drawn out, as for closing, until only a very narrow tube
remains at _C_, the end of _C_ is closed the area round _C_ is
carefully blown into shape, so that by melting off _C_ the tube _A_ will
be left with a well-rounded end. A small bulb of glass is next blown on
_A_ at _B_. This bulb must be of slightly greater diameter than the
contracted end _E_ of the larger tube (II.), so that _B_ will just fail
to pass through _E_. The length from _B_ to _C_ must not be made greater
than from _E_ to _G_ on the outside tube. The end at _C_ is then to be
cut off so as to leave a pin-hole in the end of _A_.

[Illustration: FIG. 19.]

The outer tube (II.), whose diameter may be 5 or 6 mm. greater than that
of _A_, is prepared by sealing a side tube on it at _F_, after
previously contracting the end _E_. For this purpose the end _E_ should
be closed and rounded, and then re-heated and blown out till the bulb
bursts. To ensure that the diameter of the opening is less than that of
the tube, care must be taken not to re-heat too large an area of the end
before blowing it out. It is very important that the cross section at
_E_ shall be in a plane at right angles to the axis of the tube.

Wrap a strip of writing paper, one inch in breadth, closely round the
end of _A_ at _C_ till the tube and paper will only just pass easily
into the mouth _D_ of the outer tube, push the inner tube _A_, with the
paper upon it, into _D_, and when the paper is entirely within _D_,
withdraw _A_, and cautiously push the paper a little further into the
outer tube. Insert _A_ into _DE_ through _E_, so that the bulb _B_ is
embraced by _E_. Close _D_ with a cork. Ascertain that the paper does
not fit sufficiently tightly between the two tubes to prevent the free
passage of air, by blowing into the mouth _K_ of _A_. Air shoul