This is not true because
of the influence of galvanic action. As all soft solders consist largely
of lead, if a joint is soldered, a "galvanic couple" of lead and iron, or
of lead and zinc (when the apparatus is built of galvanised steel), is
exposed to the liquid bathing it; and since in both cases the lead is
highly electro-negative to the iron or zinc, it is the iron or zinc which
suffers attack, assuming the liquid to possess any corrosive properties
whatever. Galvanised iron which has been injured during the joint-making
presents a zinc-iron couple to the water, but the zinc protects the iron;
if a lead solder is present, the iron will begin to corrode immediately
the zinc has disappeared. In the absence of lead it is the less important
metal, but in the presence of lead it is the more important (the
foundation) metal which is the soluble element of the couple. Where
practicable, joints in an acetylene generator may safely be made by
welding or by autogenous soldering ("burning"), because no other metal is
introduced into the system; any other process, except that of riveting or
folding, only hastens destruction of the plant. The ideal method of
making joints about an acetylene generator is manifestly that of
autogenous soldering, because, as will appear in Chapter IX.
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