In the language of the chemist a helium nucleus endothermic compound giving out 1.75 X 1o" gram calories per gram when it is formed from four protons and two electrons and similarly requiring this amount of energy to dissociate it back into its com ponent parts. The energy per atom works out to be 4.6X ergs, which is nearly three times the kinetic energy of the swift a particle, of range 8.6 cm. of air, from Thorium C'. As the swift a-particle represents the greatest localisation of energy at our disposal we may confidently say that the helium nucleus is likely to withstand any agent which we may bring to bear upon it with out disrupting.
The enormous energy which, as these calculations show, would be released if we could make hydrogen nuclei combine with elec trons to form helium nuclei offers a starting point for daring specu lation as to the possibilities of atomic energy, that is, nuclear energy, as a source of power. It may be noted that this is energy which would result from building atoms, not from breaking atoms.
The disruption of the aluminium nucleus, with its release of energy, or the spontaneous disruption of the radioactive atoms may be taken as examples of the breaking of atoms with release of energy. If 4 grams, that is one-seventh of an ounce, of helium could be built up from hydrogen we should anticipate a release of nuclear energy equivalent to about a million horse power for an hour. It does not follow that this energy would be released in a
form which we know how to handle profitably—it might appear as a very penetrating radiation which would pass through all our screens set to catch it. Speculation has a free range. It has been suggested that a small rate of formation of helium from hydrogen may be responsible for the maintenance of the sun's heat.
The packing effect with heavier nuclei, as represented by the departure of the atomic weights, in terms of oxygen as 16, from whole numbers is a very important study which is being investi gated by Aston with a sensitive mass spectrograph. (See ISO TOPES.) His results can be interpreted as supporting the view that the light elements of odd atomic number all have a loosely packed outer structure which has no counterpart in the more stable nuclei. The whole question of the structure of the nucleus, the detailed nature and arrangement of its parts, is still obscure. It is beset with great difficulties, in view of which the progress that is recorded in this article, which deals almost entirely with the work that has been done since 1910, is very encouraging. If at that date the investigation of the composition and structure of a particle some Icr" cms. across had been suggested as a study for the immediate future, few would have taken the matter seriously. (See also TRANSMUTATION OF ELEMENTS.)