m of oxygen balance and cylinder. At the
top is the balance arrangement, and at the center its support. At the
left is the oxygen cylinder, with reducing valve A, rubber tube D
leading from it, F the electro-magnet which opens and closes D, K the
hanger of the cylinder and support for the magnet, R the lever which
operates the supports for the cylinder and its counterpoise S, T' a box
which is raised and lowered by R, and T its surrounding box.]

It is highly desirable to determine the oxygen to within 0.1 gram, and
we are fortunate in having a balance of the type used frequently in this
laboratory which will enable us to weigh this cylinder accurately with a
sensitiveness of less than 0.1 gram. Since 1 liter of oxygen weighs 1.43
grams, it can be seen that the amount of oxygen introduced into the
chamber can be measured by this method within 70 cubic centimeters.
Even in experiments of but an hour's duration, where the amount of
oxygen admitted from the cylinder is but 25 to 30 grams, it can be seen
that the error in the weighing of the oxygen is much less than 1 per
cent.

The earlier forms of cylinders used were provided with valves which
required some special control and a rubber bag was attached to provide
for any sudden rush of gas. The construction of the valve and valve-stem
was unfortunately such that the well-known reduction valves could not be
attached without leakage under the high pressure of 120 atmospheres.
With the type of cylinder at present in use, such leakage does not occur
and therefore we simply attach to the oxygen cylinder a reduction-valve
which reduces the pressure from 120 atmospheres to about 2 or 3 pounds
to the square inch. The cylinder, together with the reduction valve, is
suspended on one arm of the balance. The equipment of the arrangement is
shown in fig. 31. (See also fig. 5, page 4.) The cylinder is supported
by a clamp K hung from the balance arm, and the reduction-valve A is
shown at the top. The counterpoise S consists of a piece of 7-inch pipe,
with caps at each end. At a convenient height a wooden shelf with
slightly raised rim is attached.

In spite of the rigid construction of this balance, it would be
detrimental to allow this enormous weight to remain on the knife-edges
permanently, so provision is made for raising the cylinders on a small
elevator arrangement which consists of small boxes of wood, T, into
which telescope other boxes, T'. A lever handle, R, when pressed
f