CHEMISTRY: HARKINS AND WILSON 
281 
ments exist, is evident when it is considered that the isotopes of lead, 
radium B, and lead from radium, differ in atomic weight by eight, or by 
4%, and even in the case of some of the hghter elements that a part of 
the irregularity may be due to the existence of isotopic forms is indi- 
cated by the discovery by Thomson and Aston that neon, formerly sup- 
posed to have an atomic weight of 20.2, consists of two isotopes of 
atomic weights 20 and 22. It has been proved by Fajans and others, 
as has been mentioned, that the helium system holds for the heavy 
radioactive elements; in this paper it is shown that it also holds for the 
27 Hghter elements, which would seem to make it almost certain that it 
holds for the other elements when the proper allowances are made for 
variations in packing effect, and differences of series. 
For the elements from helium to iron the average value of the packing 
effect is 0.78 units, which is, so far as the accuracy of the atomic weights 
allows us to determine, the same as the decrease in weight which would 
occur if four hydrogen atoms were to be transformed into one of helium. 
This means that if any element of higher atomic weight is built up 
entirely of helium atoms, then, on the average, the decrease in mass 
is practically entirely due to the primary formation of the hehum 
atoms, and not at all to the aggregation of these into the heavier atoms. 
From this point of view an atom composed entirely of hehum units 
would have extreme instability in relation to its disintegration into 
helium units, in comparison with its instabihty with reference to a 
hydrogen decomposition. Such an atom in a radioactive transforma- 
tion should lose alpha particles very much more readily than hydrogen 
nuclei, in fact, if it is remembered that even the alpha decomposition 
does not occur appreciably for many of the known elements, it will be 
seen that it is doubtful if such an atom would ever give detectable 
hydrogen disintegration. 
If the elements of higher atomic weight are built up exactly accord- 
ing to the special system presented in the table, according to which the 
members of the even numbered groups are in general aggregates of he- 
lium alone, then since all of the radioactive elements which are now 
known to give a simple alpha disintegration, belong to even numbered 
groups, they could not be expected to give hydrogen upon disintegration. 
Thus one of the chief objections to the theory that the atoms are hy- 
drogen complexes, which is based on the fact that up to the present 
time no hydrogen has been detected as the product of any radioactive 
change, is seen to be not contrary to, but rather in accord with the 
theory as here presented. However, the value of this idea is lessened 
by the fact that there is no evidence that the exact relations between 
