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. of this book, the most convenient and efficient apparatus for performing the operation is the oxy-acetylene blow-pipe, which can be employed so as to convert two separate pieces of similar metal into one homogeneous whole. In less critical situations in an acetylene plant, such as the partitions of a carbide container, &c., where the collapse of the seam or joint would not be followed by any of the effects previously suggested, there is less cause for prohibiting the use of unfortified solder; but even here, two or three rivets, just sufficient to hold the metal in position if the solder should give way, are advisedly put into all apparatus. In other portions of an acetylene installation where a merely soldered joint is exposed to warm damp gas which is in process of cooling, instead of being bathed in hard water, an equal, though totally dissimilar, danger is courted. The main constituent of such solders that are capable of being applied with the bitt is lead; lead is distinctly soluble in soft or pure water; and the water which separates by condensation out of a warm damp gas is absolutely soft, for it has been distilled. If condensation takes place at or near a soldered joint in such a way that water trickles over the solder, by slow degrees the metallic lead will be dissolved and removed, and eventually a time will come when the joint is no longer tight to gas. In fact, if an acetylene installation is of more than very small dimensions, _e.g._, when it is intended to supply any building as large as, or larger than, the average country residence, if it is to give satisfaction to both constructor and purchaser by being quite trustworthy and, possessed of a due lease of life, say ten or fifteen years, it must be built of stouter materials than the light sheets which alone are suitable for manipulation with the soldering-iron or for bending in the ordinary type of metal press. Sound cast-iron, heavy sheet-metal, or light boiler-plate is the proper substance of which to construct all the important parts of a generator, and the joints in wrought metal must be riveted and caulked or soldered autogeneously as mentioned above. So built, the installation becomes much more costly to