hydrogen as the unit, the atomic weights of nearly all the elements up to potassium fall just short of whole numbers; this indicates that there has been a diminution of mass in the evolution of these elements. A diminution of mass means a liberation of energy proportional to it, so that the amount of energy liberated in the formation of these lighter elements will be proportional to the defect of this atomic weight from the nearest integer.
Of the lighter elements whose atomic weights have been determined with great accuracy, magnesium and silicon seem to be the only ones where there are indications of an increase of mass, and in this case the increase is so slight that a very small error in the determination of the atomic weight would account for these apparent exceptions.
There are indications that some radical change in the way in which the atom is built up from the primordial atom occurs when we get to atomic weights about 40 or thereabouts. Up to this stage the atomic weights are expressed by very simple numerical relations which fail for the heavier elements; it is at this stage too that on Mendeléeff's system it is necessary to change from the short period of eight elements, which was sufficient to represent the cycle of properties of the lighter elements, to the larger one of sixteen elements to represent those of the heavier ones.
One of the most interesting results of the determination of the number of electrons in the atoms is the simplicity from one point of view of the hydrogen atom, in which there is only one negative electron. Thus, this atom is made up of an electron and the equivalent positive charge. Looked at from this point of view, the hydrogen atom is a very simple structure, in fact the simplest that could be built up of electrons and positive