\ \ =)2- which show an unduly high rate of loss among the attacking instruments—uniess, as we have seen, the target is so important as to warrant high ratios of loss provided one or a few missiles penetrate to it. In these respects the effects of scarcity in the explosive materials are intensified by the fact that it requires certain minimum amounts to produce an explosive reaction and that the minimum quantity is not likely to be reduced materially, if at all, by further research.>~ The ultimate physical limitation on world atomic bomb production is of- course the amount of ores available for the derivation of materials capable of spontaneous atomic fission, The only basic material thus far used to produce bombs oc is uranium, and for the moment only uranium need be consideredi<” * ~ oan we Ustimates of the amount of uranium available in the earth's crust vary be“Le tween 4 and 7 parts per million--a very considerable quantity indeed. The element is very widely distributed, there being about 2 ton of it present in each cubic mile of sea water and about one-seventh of an ounce per ton (average) in all granite and basalt rocks, which together comprise about 95 por cont by weight of the earth's crust, There is more uranium present in the earth's crust than cadmium, bismuth, silver, mercury, or iodine, anda it is about one thousand times as prevalent as gold, Hovwevor, the number of places in which uranium is knovmm to 31. The ficure for critical minimum inass is serret According to the Smyth Heport, it was predicted in May 191 that the critical mass would be found to Lie between - 2 ke and 100 kg (paragraph 4.9), and it was later found to be much nearer the minimim prodicted than the maximum, It is worth noting, too, that not only does the critical mass present a lowor limit in bomb size, but also that it is not feasible to use very much more than the critical mass, One reason is the dctonating problem. Hasses above the critical level cannot be kept from cxplodins, and detonation is thorefore produced by the instantancous assombly of subcritical masses. The necessity for instant and simultancous assombly of tht masscs used must obviously limit their number. The scientific oxplanation of the critical mass condition is presented in the Smyth Report in paragraphs 2.3, 2.6, and 2.7.. One must always distinguish, hovever, betircen the chain reaction which occurs in the plutoniumproducing pile and that which occurs in the bomb. Although the goneral principles determining critical mass arc similar for the two reactions, the actual mass needed ond the character of the reaction are very different in the two cases. Scc also ibid., paragraphs 2.35, 4.15-17, and 12.13-15. ar