[Illustration:  PUMPING MELTED WHITE PHOSPHORUS INTO HAND GRENADES FILLED WITH WATER—­EDGEWOOD ARSENAL]

[Illustration:  FILLING SHELL WITH “MUSTARD GAS”

Empty shells are being placed on small trucks to be run into the filling chamber.  The large truck in the foreground contains loaded shell]

For smaller work thermit has two rivals, the oxy-acetylene torch and electric welding.  The former has been described and the latter is rather out of the range of this volume, although I may mention that in the latter part of 1918 there was launched from a British shipyard the first rivotless steel vessel.  In this the steel plates forming the shell, bulkheads and floors are welded instead of being fastened together by rivets.  There are three methods of doing this depending upon the thickness of the plates and the sort of strain they are subject to.  The plates may be overlapped and tacked together at intervals by pressing the two electrodes on opposite sides of the same point until the spot is sufficiently heated to fuse together the plates here.  Or roller electrodes may be drawn slowly along the line of the desired weld, fusing the plates together continuously as they go.  Or, thirdly, the plates may be butt-welded by being pushed together edge to edge without overlapping and the electric current being passed from one plate to the other heats up the joint where the conductivity is interrupted.

It will be observed that the thermit process is essentially like the ordinary blast furnace process of smelting iron and other metals except that aluminum is used instead of carbon to take the oxygen away from the metal in the ore.  This has an advantage in case carbon-free metals are desired and the process is used for producing manganese, tungsten, titanium, molybdenum, vanadium and their allows with iron and copper.

During the war thermit found a new and terrible employment, as it was used by the airmen for setting buildings on fire and exploding ammunition dumps.  The German incendiary bombs consisted of a perforated steel nose-piece, a tail to keep it falling straight and a cylindrical body which contained a tube of thermit packed around with mineral wax containing potassium perchlorate.  The fuse was ignited as the missile was released and the thermit, as it heated up, melted the wax and allowed it to flow out together with the liquid iron through the holes in the nose-piece.  The American incendiary bombs were of a still more malignant type.  They weighed about forty pounds apiece and were charged with oil emulsion, thermit and metallic sodium.  Sodium decomposes water so that if any attempt were made to put out with a hose a fire started by one of these bombs the stream of water would be instantaneously changed into a jet of blazing hydrogen.