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and a light element. Fast neutrons lose their energy in colliding with
 
atoms of a light element in much the same way as a billard ball loses 
 
velocity in a collision with another ball. At present it is an open 
 
question whether such a chain reaction can also be made to work with 
 
fast neutrons which are not slowed down.
 
              There is reason to believe that, if fast neutrons could be used, 
 
it would be easy to construct extremely dangerous bombs. The destructive 
 
power of these bombs can only be roughly estimated, but there is no doubt 
 
that it would go far beyond all military conceptions. It appears likely 
 
that such bombs would be too heavy to be transported by airplane, but
 
still they could be transported by boat and exploded in port with 
 
disastrous results.
 
              Although at present it is uncertain whether a fast neutron 
 
reaction can be made to work, from now on this possibility will have to 
 
be constantly kept in mind in view of its far-reaching military conse-
 
quences. Experiments have been devised for settling this important point, 
 
and it is solely a question of organization to ensure that such experi-
 
ments should be actually carried out.
 
              Should the experiments show that a chain reaction will work with 
 
fast neutrons, it would then be highly advisable to arrange among scientists 
 
for withholding publications on this subject. An attempt to arrange for 
 
withholding publications on chain reactions has already been made early in 
 
March, but was abandoned in spite of favorable response in this country 
 
and in England on account of the negative attitude of certain French 
 
laboratories. The experience gained in March would make it possible to 
 
revive this attempt whenever it should be necessary.
 
                               Leo Szillard
                 (Signed)