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94 OPTICAL PROJECTION disarranged by the shock, so as not to resist a second one in the same degree. This has been proved experimentally; and therefore these chambers should always be examined, and if necessary refilled with fresh-sifted material, after any notice- able ' snap' of a jet, or at reasonably short intervals even when none may have been noticed. Also it is to be observed that, as the violence of an explosion will depend upon the amount of gas in front of the chambers, these should in all cases be used as close to the nipples as possible. They should never be used at the supply nozzles of the dissolver, but on the nozzles.of the jet; and where this light is habitually used, the safety chamber should be arranged under the nipple itself, with only a very small empty chamber above. Here also it should be constantly examined. This makes the only real * safety' jet I know; but the obstruction to the flow is great. This, however, can now be overcome by the higher pressure at command from cylinders. The other precaution was to add gas-chambers above the tank—one from which the oxygen passed to the surface of the ether, another into which the vapour passed before leaving by the exit-tube. Thus, if the tank were jarred or a little tilted, ether would not run back, but be stopped in these chambers. At the end of 1887, however, another explosion occurred with a tank thus constructed, under circumstances which for a time baffled speculation. The tank was purchased in Feb. 1887, and worked well the rest of that season; but when got out in Nov. 1877, it behaved differently. Before any pure oxygen was turned on at the jet, the latter heated the lime red hot; and this got worse and worse, until finally a severe explosion ruptured the tank, in spite of two pumice chambers attached to the dissolver. The probable reasons for the explosion passing these have already been pointed out; but the explosive condition of the tank itself puzzled everyone. It was carefully examined by several, including myself; but none could discover any apparent leakage which might have