ygen ready
prepared in bottles, and will not have to undergo the annoyance of
filling a bag. If, however, a bag is used, and it has some advantages
(the valves of bottles being generally stiff), I find that a pressure
produced by placing about two hundredweight (conveniently divided into
four fifty-six pound weights) on bags measuring 3' x 2'6" x 2' (at the
thicker end) does very well. To fill such a bag with oxygen, about
700 grms of potassium chlorate is required.

If the experimenter desires to keep his bag in good order, he must
purify his oxygen by washing it with a solution of caustic soda, and
then passing it through a "tower" of potash or soda in sticks, and,
finally, through a calcium chloride tower. This purifying apparatus
should be permanently set up on a board, so that it may be carried
about by the attendant to wherever it is required. Oxygen thus
purified does not seem to injure a good bag--at least during the
first six or seven years:

In order to reduce the annoyance of preparing oxygen, the use of the
usual thin copper conical bottle should be avoided. The makers of
steel gas bottles provide retorts of wrought iron or steel for
oxygen-making, and these do very well. They have the incidental
advantage of being strong enough to resist the attacks of a servant
when a spent charge is being removed.

The form of retort referred to is merely a large tube, closed at one
end, and with a screw coupling at the other; the dimensions may be
conveniently about 5 inches by 10. The screw threads should be filled
with fireclay (as recommended by Faraday) before the joint is screwed
up. Before purchasing a bottle the experimenter will do well to
remember that unless it is of sufficiently small diameter to go into
his largest vice, he will be inconvenienced in screwing the top on and
off. Why these affairs are not made with union joints, as they should
be, is a question which will perhaps be answered when we learn why
cork borers are still generally made of brass, though steel tube has
long been available.

Fig. 4.

These little matters may appear very trivial--and so they are--but
the purchaser of apparatus will generally find that unless he looks
after details himself, they will not be attended to for him. Whether a
union joint is provided or not, let it be seen that the end of the
delivery tube is either small enough to fit a large rubber tube
connection going to the wash-bottle, or large enough to a