In some of the very badly designed generators
of a few years back this tarry matter was distinctly visible when the
apparatus was disconnected for recharging, for the spent carbide was
exceptionally yellow, brown, or blackish in colour, [Footnote: As will be
pointed out later, the colour of the spent lime cannot always be employed
as a means for judging whether overheating has occurred in a generator.]
and the odour of tar was as noticeable as that of crude acetylene.
There is another effect of heat upon acetylene, more calculated to be
dangerous than any of those just mentioned, which must not be lost sight
of. Being an endothermic substance, acetylene is prone to decompose into
its elements--
(7) C_2H_2 -> C_2 + H_2
whenever it has the opportunity; and the opportunity arrives if the
temperature of the gas risen to 780 deg. C., or if the pressure under which
the gas is stored exceeds two atmospheres absolute (roughly 30 lb. per
square inch). It decomposes, be it carefully understood, in the complete
absence of air, directly the smallest spark of red-hot material or of
electricity, or directly a gentle shock, such as that of a fall or blow
on the vessel holding it, is applied to any volume of acetylene existing
at a temperature exceeding 780 deg.
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