all tubes. For this purpose some
"pure" sulphuric acid which has been boiled with pure ammonium
sulphate is placed in each thistle funnel, and when the fall tube is
dirty the connection to the mercury supply is cut off at the pinch
cock so as to leave the tube between this entry and the head of the
fall tube quite full of mercury, and the sulphuric acid is allowed to
run down the fall tube by raising the stopper. The fall tube should
be allowed to stand full of acid for an hour or so, after which it
will be found to be fairly clean.

Of course the mercury reservoir thus obtains a layer of acid above the
mercury, and as it is better not to run the risk of any acid getting
into the pump except in the fall tubes, the reservoir is best emptied
from the bottom, by a syphon, if a suitable vessel cannot be procured,
so that clean mercury only is withdrawn.

The phosphorus pentoxide tube is best made as shown simply from a bit
of wide tube, with two side connections fused to the rest of the pump.
It is no more trouble to cut the tube and fuse it up again when the
drying material is renewed than to adjust the drying tube to two fixed
stoppers, which is the alternative. The practice here recommended is
rendered possible only by the oxygas blow-pipe with hooked nozzle.
The connection between the pump and tube to be exhausted is made
simply by a short bit of rubber tube immersed in mercury.

The phosphorus pentoxide should be pure, or rather free from
phosphorus and lower oxides; unless this be the case, the vapour
arising from it is apt to soil the mercury in the pump. The
phosphorus pentoxide is purified by distilling with oxygen over
red-hot platinum black; if this cannot be done, the pentoxide should
at least be strongly heated in a tube, in a current of dry air or
oxygen, before it is placed in the drying tube.

The mercury used for the pump must be scrupulously clean. It does
not, however, require to have been distilled in vacuo. It is
sufficient to purify it by allowing it to fall in a fine spray into a
large or rather tall jar of 25 per cent nitric acid and 75 per cent
water. The mercury is then to be washed and dried by heating to, say,
110 deg. C. in a porcelain dish.

Exhausting a Roentgen Tube.

With a pump such as has been described there is seldom any advantage
in fusing an extra connection to the vacuum tube so as to allow of a
preliminary exhaustion by means of a water pump. About half an hour's
pu