Greater legibility will be
obtained by staining the water with a few drops of caramel solution, or
of indigo sulphate (indigo carmine); or, in the absence of these dyes,
with a drop or two of common blue-black writing ink. If they are not
erected in perfectly frost-free situations, the gauges may be filled with
a mixture of glycerin and pure alcohol (not methylated spirit), with or
without a certain proportion of water, which will not freeze at any
winter temperature. The necessary mixture, which must have a density of
exactly 1.00, could be procured from any pharmacist.
It is the pressure as indicated by the pressure gauge which is referred
to in this book in all cases where the term "pressure of the gas" or the
like is used. The quantity of acetylene which will flow in a given time
from the open end of a pipe is a function of this pressure, while the
quantity of acetylene escaping through a tiny hole or crack or a burner
orifice also depends on this total pressure, though the ratio in this
instance is not a simple one, owing to the varying influence of friction
between the issuing gas and the sides of the orifice. Where, however,
acetylene or other gas is flowing through pipes or apparatus there is a
loss of energy, indicated by a falling off in the pressure due to
friction, or to the performance of work, such as actuating a gas-meter.
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