The nuclear age in which the human race is living, and may soon be dying, began for the ge

neral public with the dropping of an atom bomb on Hiroshima on 6 August 1945. But for nuclear scientists and for certain American authorities, it had been known for some time that such a weapon was possible. Work towards making it had been begun by the United States, Canada and Britain very soon after the beginning of the Second World War. The existence of possibly explosive forces in the nuclei of atoms had been known ever since the structure of atoms was discovered by Rutherford.

An atom consists of a tiny core called the "nucleus" with attendant electrons circling round it. The hydrogen atom, which is the simplest and lightest, has only one electron. Heavier atoms have more and more as they go up the scale. The first discovery that had to do with what goes on in nuclei was radioactivity, which is caused by particles being shot out of the nucleus. It was known that a great deal of energy is locked up in the nucleus, but, until just before the outbreak of the Second World War, there was no way of releasing this energy in any large quantity. A revolutionary discovery was that, in certain circumstances, mass can be transformed into energy in accordance with Einstein's formula which states that the energy generated is equal to the mass lost multiplied by the square of the velocity of light.

The A-bomb, however, used a different process, depending upon radioactivity. In this process, called "fission", a heavier atom splits into two lighter atoms. In general, in radioactive substances this fission proceeds at a constant rate which is slow where substances occurring in nature are concerned. But there is one form. of uranium called "U235" which, when it is pure, sets up a chain reaction which spreads like fire, though with enormously greater rapidity. It is this substance which was used in making the atom bomb.

The political background of the atomic scientists' work was the determination to defeat the Nazis. It was held--I think rightly--that a Nazi victory would be an appalling disaster. It was also held, in Western countries, that German scientists must be well advanced towards making an A-bomb, and that if they succeeded before the West did they would probably win the war. When the war was over, it was discovered, to the complete astonishment of both American and British scientists, that the Germans were nowhere near success, and as everybody knows, the Germans were defeated before any nuclear weapons had been made. But I do not think that nuclear scientists of the West can be blamed for thinking the work urgent and necessary. Even Einstein favored it.

When, however, the German war was finished, the great majority of those scientists who had collaborated towards making the A-bomb considered that it should not be used against the Japanese, who were already on the verge of defeat and, in any case, did not constitute such a menace to the world as Hitler. Many of them made urgent representations to the American Government advocating that, instead of using the bomb as a weapon of war, they should after a public announcement, explode it in a desert, and that future control of nuclear energy should be placed in the hands of an international authority. Seven of the most eminent of nuclear scientists drew up what is known as "The Franck Report" which they presented to the Secretary of War in June 1945. This is a very admirable and far-seeing document, and if it had won the assent of the politicians, none of our subsequent terrors would have arisen.

We may infer that the writer's attitude towards the A-bomb is that ______.

A．it is a necessary evil

B．it is a terrible threat to the whole of mankind

C．it played a vital part in defeating the Japanese

D．it was a wonderful invention