f the tube can be arrived
at so long as the discharge is kept going continually. When the spark
would rather go over an inch of air in the spark gap than through the
tube the pumping and sparking may be interrupted and the tube allowed
to rest for, say, five minutes. It will generally be found that the
equivalent resistance of the tube will be largely increased by this
period of quiescence. It may even be found that the spark will now
prefer to pass an air gap 3 inches long.

In any case the sparking should now be continued, the pump being at
rest, and the variations of tube resistance watched by adjusting the
spark gap. If the resistance falls below an equivalent of 2 inches of
air in the gap the pump must be brought into action again and
continued until the resistance as thus estimated remains fairly
constant for, say, ten minutes. When this occurs the narrow neck of
the exhaust tube may be strongly heated till the blow-pipe flame
begins to show traces of sodium light. The flame must then be
withdrawn and the discharge again tested. This is necessary because
it occasionally happens that gas is given off during the heating of
the neck to the neighbourhood of its fusion temperature.

If all is right the neck may now be fused entirely off and the tube is
finished. Tubes of the focus pattern with large platinum anodes are
in general (in my experience) much more difficult to exhaust than
tubes of the kind first described. This is possibly to be attributed
mainly to the gas given off by the platinum, but is also, no doubt,
due to the tubes being much larger and exposing a larger glass
surface. The type of tube described first generally takes about two
hours to exhaust by a pump made as explained, while a "focus" tube has
taken as long as nine hours, eight of which have been consumed after
the tube was exhausted to the hammering point.

The pressure at which the maximum heating of the anode by the cathode
rays occurs is a good deal higher than that at which the maximum
Roentgen effect is produced. There is little doubt that the Roentgen
radiation changes in nature to some extent as the vacuum improves
either as a primary or secondary effect. It is therefore of some
importance to test the tube for the purpose for which it is to be used
during the actual exhaustion. It has been stated, for instance, that
the relative penetrability of bone and flesh to Roentgen radiation
attains a maximum difference at a certain