McCie.ianp anp Hackerr—Secondary Radiation from Compounds. 33 
FurrHer DETERMINATIONS FOR COMPOUNDS. 
The results given in the above table show clearly that the emission of secondary 
radiation of the B-ray type is an additive atomic property, and can be calculated 
for any molecule when we know the radiating powers of its constituents. 
In addition to the examination of the compounds contained in this table, a 
number of other determinations were made. In the first place, some com- 
pounds rich in hydrogen, such as water, toluene, benzene, and aniline, were 
tested in order to determine the radiating power of hydrogen, the element of 
lowest atomic weight. In these determinations the value of « for the compound 
and for its constituents, other than hydrogen, was known, and the value for 
hydrogen could therefore be calculated. This method was justified, since the 
previous results showed that the property under examination was strictly additive. 
Some of the compounds tested gave a very small positive value of « for hydrogen, 
while others gave a small negative value, showing that the true value for hydro- 
gen was too small to be measured accurately by this method. This is exactly 
what was to be expected from the results in the previous paper, which showed 
that the secondary radiation from elements diminishes as the atomic weight 
diminishes. 
Since the compounds hitherto examined had a rather simple molecular 
structure, a few very complex compounds were examined, such as potassium 
ferrocyanide, potassium ferricyanide, and chrome alum. In every case the 
agreement between the observed and the calculated values was quite as good as 
in the case of the simple compounds contained in the above table. 
Another application of the results in this paper has reference to the previous 
work by one of us on the variation of the secondary @ radiation with the atomic 
weight. It was shown in the previous paper that while the secondary radiation 
always increases when the atomic weight increases, the rate of increase is such as 
to give a division of the elements exactly corresponding to the division into 
‘“periods” employed in chemistry. Among the elements previously tested, all the 
chemical periods were represented except the one containing barium, cerium, 
didymium, &c. On account of the difficulty of obtaining these elements in 
sufficient quantity in the pure state, none of them were previously tested. It is 
now possible to calculate the secondary radiation for the element, knowing it for 
a compound containing the element and for the other constituents. Experiments 
have therefore been made with compounds of barium, cerium, and didymium. 
The curves given in the previous paper, and which are repeated below (fig. 2), 
showed that as we pass along a chemical period in the direction of increasing 
atomic weight, there is a rapid increase of secondary radiation, with a marked 
