Their disadvantages
are twofold, though these may be avoided or circumvented: in all types
save one the carbide is in excess at the immediate place and time of
decomposition; and in all types without exception the carbide in the
whole of the generator is in excess, so that the phenomenon of "after-
generation" occurs with more or less severity. As explained in the last
chapter, after-generation is the secondary production of acetylene which
takes place more or less slowly after the primary reaction is finished,
proceeding either between calcium hydroxide, merely damp lime, or damp
gas and calcium carbide, with an evolution of more acetylene. As it is
possible, and indeed usual, to fit a holder of some capacity even to an
automatic generator, the simple fact that more acetylene is liberated
after the main reaction is over does not matter, for the gas can be
safely stored without waste and entirely without trouble or danger. The
real objection to after-generation is the difficulty of controlling the
temperature and of dissipating the heat with which the reaction is
accompanied. It will be evident that the balance of advantage, weighing
mechanical simplicity against chemical superiority, is somewhat even
between carbide-to-water and water-to-carbide generators of the proper
type; but the balance inclines towards the former distinctly in the ease
of non-automatic apparatus, and points rather to the latter when
automatism is desired.
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