A flexible capillary lead was connected to the needle valve, F, above the valve seat, and led to a mercury manometer stationed on the wall of the dark room.A capillary brass nipple taking off from the lateral projection, 6, with a ball and socket joint, was connected to needle valve, H, above its seat with a screw fitting and metallic seat.Needle valve, H, below its valve seat, was connected to needle valve, I, below its valve seat, by a capillary brass nipple with screw fittings and metallic seats. A flexible capillary tube leading from a common mapifold used for charging oxygen and nitrogen, was connected to needle valve, I, below its valve seat with a ball and socket joint. The needle valve, I, was supplied with an opening above its valve seat, 7, for flushing the line before charging the bomb. Pressure Element-For recording time-pressure curves of the explosions a modified Midgley indicator was used. The pressure element, J, similar to that described by Midgley,7

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perience to sharpen properly and keep the knife in condition. These considerations have led the present writers to publish the result of a study of the polishing methods that has been in progress for some time, and in which has been developed a technic that is rapid, can be successfully carried out with any polishing equipment, and leads to the formation of ideal surfaces for microscopical examination.Figures 11, 12, 13, and 14 illustrate the appearance of the structures of a 1 per cent antimony-lead alloy after various treatments, as developed by the methods of polishing and etching described here. Figures 15 and 16 illustrate the structure of a 3 per cent tin-lead alloy as developed by the same process.The time required to prepare a sample depends entirely on the skill of the operator, an experienced one being able to complete the entire operation in less than an hour. The structure of properly prepared samples can be easily developed in 10 seconds, even when employing as slow an acting reagent as that previously described. It is by no means necessary to etch deeply in order to get below polishing imperfections.The fact that the blackening often observed during the polishing operation can be avoided by careful manipulation or removed, when it appears, by gentle polishing on a very wet cloth, seems to indicate that its cause is other than oxidation due to the polishing operation. It has been observed that a polished specimen of lead will retain its bright surface for several days.