Why, one may ask, does not spontaneous combustion of U-235 take place in nature? Why, indeed, has not all the U-235 in nature caught fire automatically long ago? To this also there is a simple answer. Just as in the spontaneous combustion of sawdust the material must be dry enough to burn, so must the U-235. Only in place of the word “dry” we must use the word “concentrated.” The U-235 found in nature is very much diluted with another element that makes it “wet.” It therefore must be separated first, by a very laborious and costly process, from the nonfissionable, or “wetting,” element. Even then it won’t catch fire, and could not be made to burn by any means, until the amount separated (“dried”) reaches the critical mass. When these two conditions—conditions that do not exist in nature—are met, the U-235 catches fire just as sawdust does when it reaches the critical temperature. The fact that as soon as a critical mass is assembled the three elemental atomic fuels burst into flame automatically thus puts a definite limit to the amount of material that can be used in the conventional A-bomb. The best you can do is to incorporate into a bomb two fragments, let us say, of nine tenths of a critical mass each. To enclose more than two such fragments would present difficulties that appear impossible to overcome. It is this limitation of size, an insurmountable roadblock put there by mother nature, that makes the basic difference between the A-bomb and the H-bomb. For, as we have already seen, to light an atomic fire with deuterium it is necessary to strike a match generating a flame with a temperature of about 50,000,000 degrees centigrade. As long as no such match is applied, no fire can start. It thus becomes obvious that deuterium is not limited by nature to a critical mass. A quantity of deuterium a thousand times the amount of the U-235, and hence a thousand times more powerful, can therefore be incorporated in an ordinary A-bomb, where it would remain quiescent until the A-bomb match is struck. Weight for weight, deuterium has only a little more energy content than U-235, so that a bomb incorporating a 1,000 kilograms (one ton) of deuterium would thus have an energy of 20,000,000 tons of TNT.