-l5fore has been extremely limited and only the richer deposits were worth working-—-— mainly for their vanadium or radium content--or for that matter worth keeping track of 38 go far as uranium itself was concerned, no ticular encouragement for prospecting existed, It is true that the radioactivity of uranium affordga yéry sensitive test of its presence, and that the data accumulated over the last fifty years make it appear rather unlikely that wholly new deposits will be found comparable to those of Canada or the Congo, But it is not wnlikely that in those regions known to contain uranium, further exploration will reveal much larger quantities than had previously been suspected. It seems hardly conceivable, for example, that in the great expanse of European and Asiatic Russia no additional workable deposits will be discovered. In that connection it is worth noting that the cost of mining the ore and of extracting the uranium is so small a fraction of the cost of bomb production that (as is not true in the search for radium) even poorer deposits are decidedly usable, Within certain wide limits, in other words, the relative richness of the ore is not critical, In fact, as much uranium can be obtained as the nations of | the world really desire. Gold is commonly mined from ores containing only one=- fifth of an ounce per ton of rock, and there are vast quantities of granite which contain from one-fifth to one ounce of uranium per ton of rock. Although the American experiment has thus far been confined to the use of uranium, it should be noted that the atoms of thorium and protoactinium also undergo fission when bombarded by neutrons. Protoactinium can be eliminated from consideration because of its scarcity in nature, but thorium is even more plentiful than uranium, its average distribution in the earth's crust being some. twelve parts per million, 8. Fairly high concentrations of thorium oxide are | "Material for U-235," The Economist (London), November 3, 1945, pp. 629-30. +