From chemical evidence, Hahn and Strassmann conclude that radioactive barium nuclei (atom number Z = 56) are produced when uranium (Z = 92) is bombarded by neutrons. It has been pointed out that this might be explained as a result of a "fission" of the uranium nucleus, similar to the division of a droplet into two. The energy liberated in such processes was estimated to be about 200 Mev, both from mass defect considerations and from the repulsion of the two nuclei resulting from the "fission" process.

If this picture is correct, one would expect fast-moving nuclei of atomic number 40 to 50 and atomic weight 100 to 150, and up to 100 Mev energy, to emerge from a layer of uranium bombarded with neutrons. In spite of their high energy, these nuclei should have a range in air of a few millimeters only, on account of their high effective charge (estimated to be about 20), which implies very dense ionization. Each such particle should produce a total of about 3 million ion pairs.

By means of a uranium-lined ionization chamber, connected to a linear amplifier, I have succeeded in demonstrating the occurrence of such bursts of ionization. The amplifier was connected to a thyratron which was biased so as to count only pulses corresponding to at least 5 x 105 ion pairs. About 15 particles per minute were recorded when 300 milligram of radium, mixed with beryllium, was placed one centimeter from the uranium lining. No pulses at all were recorded during repeated check runs of several hours total duration when either the neutron source or the uranium lining was removed. With the neutron source at a distance of four centimeters from the uranium lining, surrounding the source with paraffin wax enhanced the effect by a factor of two.

It was checked that the number of pulses depended linearly on the strength of the neutron source; this was done in order to exclude the possibility that the pulses are produced by accidental summation of smaller pulses. When the amplifier was connected to an oscillograph, the large pulses could be seen very distinctly on the background of much smaller pulses due to the alpha particles of uranium.

By varying the bias of the thyratron, the maximum size of pulses was found to correspond to at least 2 million ion pairs, or an energy loss of 70 Mev of the particle within the chamber. Since the longest path of a particle in the chamber was 3 centimeters, and the chamber was filled with hydrogen at atmospheric pressure, the particles must ionize so heavily that they can make 2 million ion pairs on a path equivalent to 0.8 cm of air or less. From this it can be estimated that the ionizing particles must have an atomic weight of at least about seventy, assuming a reasonable connection between atomic weight and effective charge. This seems to be conclusive physical evidence for the breaking up of uranium nuclei into parts of comparable size, as indicated by the experiments of Hahn and Strassmann.

Experiments with thorium instead of uranium gave quite similar results, except that surrounding the neutron source with paraffin did not enhance, but slightly diminished the effect. This gives evidence in favor of the suggestion that also in the case of thorium some, if not all of the activities produced by neutron bombardment, should be ascribed to light elements. It should be remembered that no enhancement by paraffin has been found for the activities produced in thorium, except for one which is isotopic with thorium and is almost certainly produced by simple capture of the neutron.

Meitner has suggested another interesting experiment. If a metal plate is placed close to a uranium layer bombarded with neutrons, one would expect an active deposit of the light atoms emitted in the "fission" of the uranium to form on the plate. We hope to carry out such experiments, using the powerful source of neutrons which our high-tension apparatus will soon be able to provide.

In the course of the last four months it has been made probable - through the work of Joliot in France as well as Fermi and Szilard in America - that it may become possible to set up a nuclear chain reaction in a large mass of uranium, by which vast amounts of power and large quantities of new radium-like elements would be generated. Now it appears almost certain that this could be achieved in the immediate future.

This new phenomenon would also lead to the construction of bombs, and it is conceivable - though much less certain - that extremely powerful bombs of a new type may thus be constructed. A single bomb of this type, carried by boat or exploded in a port, might well destroy the whole port together with some of the surrounding territory. However, such bombs might well prove to be too heavy for transportation by air.

A weapon of an unparalleled power is being created which will completely change all future conditions of warfare. Unless some agreement about the control of the use of the new active materials can be obtained in due time, any temporary advantage, however great, may be outweighed by a perpetual menace to human security. An initiative, aiming at forestalling a fateful competition, should serve to uproot any cause of distrust between the powers of whose harmonious collaboration the fate of coming generations will depend.

We were a true community of people working toward a common goal. I think that irrespective of what was done with it, irrespective of what was to come of it, it was clear that this was a very major change in the human situation, and the people were playing a part in history. We started out by thinking that it might make the difference between defeat and victory, and ended by thinking that it might make a difference between a world periodically convulsed by increasingly ferocious global wars and a world in which there will be none.

It had all the greatest scientists of the Western World, and something I'd never known before: they didn't care who you were, or what you were. All they cared-about was what you could contribute and what you had in the way of ideas. This was new to me. I met scientists I would never have expected to have seen in my whole life before.

We spent a lot of time and risked a lot of lives to do so. Of my little group of eight, two were killed. We were using high explosives and radioactive material in large quantities for the first time. There was a series of events that rocked us. We were working hard, day and night, to do something that had never been done before. It might not work at all. I remember working late one night with my friend Louis Slotin. He was killed by a radiation accident. We shared the job. It could have been I. But it was he, who was there that day.

James Franck, a truly wonderful man, produced the Franck Report: Don't drop the bomb on a city. Drop it as a demonstration and offer a warning. This was about a month before Hiroshima. The movement against the bomb was beginning among the physicists, but with little hope. It was strong at Chicago, but it didn't affect Los Alamos.

We heard the news of Hiroshima from the airplane itself, a coded message. When they returned, we didn't see them. The generals had them. But then the people came back with photographs. I remember looking at them with awe and terror. We knew a terrible thing had been unleashed. The men had a great party that night to celebrate, but we didn't go. Almost no physicists went to it. We obviously killed a hundred thousand people and that was nothing to have a party about. The reality confronts you with things you could never anticipate.

Before I went to Wendover, an English physicist. Bill Penney, held a seminar five days after the test at Los Alamos. He applied his calculations. He predicted that this would reduce a city of three or four hundred thousand people to nothing but a sink for disaster relief, bandages, and hospitals. He made it absolutely clear in numbers. It was reality. We knew it, but we didn't see it. As soon as the bombs were dropped, the scientists, with few exceptions, felt the time had come to end all wars.

I made the suggestion fully aware-of the difference between a first use of a new weapon in which the blood of hundreds of thousands of men, women, and children would be on our hands, and of the dreadful precedent that would thereby be set - and that of illegally retaining, even executing, if you will, a few men. I realised by the dramatic character of the statement how far I had departed from the gentle pacifist of my student days. My regret in retrospect is not for the savagery of the remark, but rather that I did not use more forceful arguments with Oppenheimer in order to influence him to take more seriously my suggestion that some kind of a demonstration be made instead of the actual use.

The military advantages and the saving of American lives achieved by the sudden use of atomic bombs against Japan may be outweighed by the ensuing loss of confidence and by a wave of horror and repulsion sweeping over the rest of the world and perhaps even dividing public opinion at home.

From this point of view, a demonstration of the new weapon might best be made, before the yes of representatives of all the United Nations, on the desert or a barren island. The best possible atmosphere for the achievement of an international agreement could be achieved if America could say to the world, "You see what sort of a weapon we had but did not use. We are ready to renounce its use in the future if other nations join us-in this renunciation and agree to the establishment of an efficient international control.

The opinions of our scientific colleagues on the initial use of these weapons are not unanimous: they range from the proposal of a purely technical demonstration to that of the military application best designed to induce surrender. Those who advocate a purely technical demonstration would wish to outlaw the use of atomic weapons, and have feared that if we use the weapons now our position in future negotiations will be prejudiced. Others emphasise the opportunity of saving American lives by immediate military use, and believe that such use will improve the international prospects, in that they are more concerned with the prevention of war than with the elimination of this special weapon. We find ourselves closer to these latter views; we can propose no technical demonstration likely to bring an end to the war; we see no acceptable alternative to direct military use.

With regard to these general aspects of the use of atomic energy, it is clear that we, as scientific men, have no proprietary rights. It is true that we are among the few citizens who have had occasion to give thoughtful consideration to the problems during the past few years. We have, however, no claim to special competence in solving the political, social, and military problems which are presented by the advent of atomic power.

The task of creating the atomic bomb had been entrusted to a special unit of the Army Corps of Engineers, the so-called Manhattan District, headed by Major General Leslie R. Groves. The primary effort, however, had come from British and American scientists, working in laboratories and offices scattered throughout the nation.

Dr. J. Robert Oppenheimer, the distinguished physicist from the University of California, had set up the key establishment in the whole process at Los Alamos, New Mexico. More than any other one man, Oppenheimer is to be credited with the achievement of the completed bomb.

My own knowledge of these developments had come about only after I became President, when Secretary Stimson had given me the full story. He had told me at that time that the project was nearing completion and that a bomb could be expected within another four months. It was at his suggestion, too, that I had then set up a committee of top men and had asked them to study with great care the implications the new weapon might have for us.

At Potsdam, as elsewhere, the secret of the atomic bomb was kept closely guarded. We did not extend the very small circle of Americans who knew about it. Churchill naturally knew about the atomic bomb project from its very beginning, because it had involved the pooling of British and American technical skill.

On July 24th I casually mentioned to Stalin that we had a new weapon of special destructive force. The Russian Premier showed no unusual interest. All he said was that he was glad to hear it and hoped we would make "good use of it against the Japanese".

The final decision of where and when to use the atomic bomb was up to me. Let there be no mistake about it. I regarded the bomb as a military weapon and never had any doubt that it should be used.

I voiced to him my grave misgivings, first on the basis of my belief that Japan was already defeated and that dropping the bomb was completely unnecessary, and secondly because I thought that our country should avoid shocking world opinion by the use of a weapon whose employment was, I thought, no longer mandatory as a measure to save American lives. It was my belief that Japan was, at that very moment, seeking some way to surrender with a minimum loss of face.

Once it had been tested, President Truman faced the decision as to whether to use it. He did not like the idea, but was persuaded that it would shorten the war against Japan and save American lives. It is my opinion that the use of this barbarous weapon at Hiroshima and Nagasaki was of no material assistance in our war against Japan. The Japanese were already defeated and ready to surrender because of the effective sea blockade and the successful bombing with conventional weapons.

It was my reaction that the scientists and others wanted to make this test because of the vast sums that had been spent on the project. Truman knew that, and so did the other people involved. However, the Chief Executive made a decision to use the bomb on two cities in Japan. We had only produced two bombs at that time. We did not know which cities would be the targets, but the President specified that the bombs should be used against military facilities.

The lethal possibilities of atomic warfare in the future are frightening. My own feeling was that, in being the first to use it, we had adopted an ethical standard common to the barbarians of the Dark Ages. I was not taught to make war in that fashion, and wars cannot be won by destroying women and children. We were the first to have this weapon in our possession, and the first to use it. There is a practical certainty that potential enemies will develop it in the future and that atomic bombs will some time be used against us.

If we had made a demonstration and that had failed, then I think dropping the bomb would have been justified in order to end the war. To drop it without warning was wrong. It was wrong on moral grounds - it killed; it was wrong, although I could not see that at the time, on practical grounds because the dropping of the bomb has distorted our views, has changed our whole outlook. We are not now looking on the accomplishment of atomic explosions as progress which can, and should, be used in the right way. We had started at that time to look at it as something horrible, something that should not be continued.

I think it was right to drop the bombs because I believe that the dropping of these bombs brought the war to a close much quicker than would have been possible otherwise. I think if people who now debate this question had seen the preparations which we were making for evacuating the wounded, the hospital preparations and everything, anticipating an actual landing, that they would have realised that we actually saved lives: not only our own soldiers' lives, but the lives of the Japanese, because had we been forced to actually attempt to occupy the island I think the death toll would have been tremendous.

The chief lesson I have learned in a long life is that the only way you can make a man trustworthy is to trust him; and the surest way to make him untrustworthy is to distrust him. If the atomic bomb were merely another, though more devastating, military weapon to be assimilated into our pattern of international relations, it would be one thing. We would then follow the old custom of secrecy and nationalistic military superiority relying on international caution to prescribe the future use of the weapon as we did with gas. But I think the bomb instead constitutes merely a first step in a new control by man over the forces of nature too revolutionary and dangerous to fit into old concepts. My idea of an approach to the Soviets would be a direct proposal after discussion with the British that we would be prepared in effect to enter an agreement with the Russians, the general purpose of which would be to control and limit the use of the atomic bomb as an instrument of war.

The bomb that hurried Russia into Far Eastern war a week ahead of schedule and drove Japan to surrender has accomplished the specific job for which it was created. From the point of view of military strategy, $2,000,000,000 (the cost of the bomb and the cost of nine days of war) was never better spent. The suffering, the wholesale slaughter it entailed, have been outweighed by its spectacular success; Allied leaders can rightly claim that the loss of life on both sides would have been many times greater if the atomic bomb had not been used and Japan had gone on fighting. There is no answer to this argument. The danger is that it will encourage those in power to assume that, once accepted as valid, the argument can be applied equally well in the future. If that assumption should be permitted, the chance of saving civilization - perhaps the world itself - from destruction is a remote one.