322 FLUIDITY AND PLASTICITY
In the measurements of plasticity it has been found that
high pressures give data which may be handled more simply
than the data at low pressures. But a multiple-tube stabilizer
to give two atmospheres of pressure is both complicated and
expensive, hence a mercury stabilizer seems desirable. However
a mercury stabilizer was not used at first because as soon as the
pressure became great enough to bubble through the mercury at
all, a large amount of gas suddenly came off causing a violent
fluctuation in the pressure. This intermittent flow of air is
partly due to the failure of the mercury to wet the tube allowing
a continuous air channel to be formed over a considerable
distance between the mercury and the tube. This difficulty
can be overcome by the amalgamation of the tube by means of
sodium amalgam. A further difficulty arose from the necessity
of keeping the volume of gas bubbling through the stabilizer as
small as possible while maintaining the flow continuously. This
trouble was completely overcome by placing a Davis-Bourneville
reducing valve at the point C of the apparatus shown in Fig. 92,
a flow indicator just between the needle-valve D and the pressure-
reservoir F, and another flow indicator between the valve E
and the mercury stabilizer.
The flow indicator consists of two similar vials connected by
an inverted TJ-tube leading to the bottom of both vials through
two-hole rubber stoppers. A little glycerol is added to one of the
vials at the start and the rate of bubbling of the gas through the
liquid serves to indicate the direction of movement of the gas
as well as its velocity.
The mercury stabilizer consists of an single iron tube of some
25 mm internal diameter into which leads the inner tube having
a diameter of 5 mm just as in the water stabilizer. The outer
tube is closed at the bottom by means of a cap but near the
bottom a side tube leads off for the attachment of a stout rubber
tube which is connected in turn with a glass receiver of about
2 liters capacity. This receiver can be raised and lowered and
hung on stout hooks provided for the purpose at frequent vertical
intervals. In order to change from one pressure to another, it is
necessary for mercury to be added to or taken from the stabilizer.
This is very easily accomplished by simply raising or lowering
the receiver. For a pressure of two atmospheres not over 10 kg