of the view that the atoms of the consecutive elements differ from each other by the addition of a primordial atom, which apparently is the atom of helium. But though the number of electrons in the atom apparently increases with perfect regularity, the mass of the atom, at any rate in the case of the heavier elements, does not do so. Thus the addition of a constant primordial atom does not produce a constant increase in the mass; there must, therefore, be a change in mass when the primordial atoms coalesce to form the atom of a chemical element; and from the values of the atomic weights of the elements we can get an indication of the change in mass which has occurred. The consideration of this point leads to some very interesting results. It is entirely in accordance with electrical principles that some change in mass should occur when these primordial atoms coalesce; we know, for example, that when we push two similarly electrified bodies together against their mutual repulsion, the mass of the two increases by an amount proportional to the work done in pushing them together. When we know the work spent or liberated in any change of condition, we can calculate the consequent increase or decrease in mass. In chemical combination heat is liberated, and there is, therefore, a change in mass, but a calculation shows that even in the cases when the greatest amount of heat is produced, as for example in the burning of coal, the change in mass is too small to be detected by our most sensitive balances, and though some chemists have devoted a lifetime to the investigation, no change in mass has ever been established as the result of chemical combination. Since the atomic weights of the elements show that in their formation a measurable change of mass has taken place, the changes of energy involved in the formation of the elements must be enormous
