neutron excess was achieved. The phenomenon of criticality is obviously an engineering embarrassment when one wishes to obtain large explosions by using large masses of the fissile elements.

2: Annihilation Reactions 
    Only one instance is known to science of the complete 100 per cent conversion of mass into energy. In 1932, during his studies on cosmic rays, Dr. C. D. Anderson discovered a particle which was of the mass of an electron but which was deflected in an opposite direction to the negative electron when placed in a magnetic field and hence was positively charged. He called this curious particle the "Positron" and it is usually represented by the symbol OE+l.
     Negative electrons are ubiquitous in nature and the lifetime of a positron which is expelled into an atomic world containing a superabundance of electrons can only be of very brief character indeed, i.e. much less than a microsecond. It is now known that electrons and positrons combine almost instantaneously, forming an intermediate complex (positronium) which is immediately transformed into two gamma ray photons which move in opposite directions and which have individual energies of 0.51 MeV, i.e. are equivalent exactly to the electron or positron mass. The probability of utilizing this annihilation reaction, following some initial system of rapid positron production, to produce an atomic device cannot be ignored but the difficulties of fabricating such an assembly would appear to be insuperably great.

3. Fusion Reactions 
    It has been indicated that the property of fission is exhibited primarily by the heavier elements and involves a nuclear disruption. Conversely the phenomenon of "fusion" is more characteristic of the lighter elements and involves the collisional synthesis of two light nuclei to form stable products with an attendant energy release. These energy releases are not of the 200 MeV per atom magnitude shown by fissile elements, but when the lesser masses of the light nuclei are taken into account, the energy yield per gram of matter is very considerable in the fusion processes also.
     Spectroscopists have known for many years that the main bulk of the sun (which has an internal temperature of 20 million degrees Centrigrade) is composed of hydrogen. Astrophysicists believe that fusion reactions involving hydrogen account for the sun's apparently inexhaustible supply of light and heat energy. There appears to be no limit to the quantity of "fusion" explosive which could be usefully employed. The scientific press has been inundated for some years with articles by various authors who suggest a variety of reactions which might yield nuclear devices of megaton type.
     Most of the preferred reactions suggest the employment of the isotopes of hydrogen. Isotopes are elements of identical chemical properties and possess the same atomic number but are of differing atomic mass. The common isotopes of hydrogen are deuterium, 1H2, and tritium, 1H3, and some of the nuclear changes in which they participate are listed in the following table: