McCLetLtanp—On Secondary Radiation and Atomic Structure. 5 
"TABLE—continued. 
Ratio of Second- | 
Substances. Secondary | Atomic Weight. ary nal Density: 
Calcium, | 55 | 40:1 1:37 1:6 
Potassium, . : 3 54 | 39°1 1:38 0:9 
Sulphur, : : ; 53°5 32°06 | 1°67 2°07 
| Phosphorus,.  . 53 31-0 171 1:8 
Aluminium, . : : 48 27°71 1-77 | 27 
Magnesium, . : ; 43 24°4 1:76 | 1:7 
Sodium, : : 2 42 23°0 1°82 | 0:97 
Carbon, : : : 35 12-0 2°91 | 1:8 
Discussion oF RESULTS. 
Throughout the table the secondary radiation is greater, the greater the 
atomic weight. It was not found possible to distinguish between bismuth and 
lead, although there is a small difference of atomic weight. 
The secondary radiation is not proportional to the atomic weight. As the 
atomic weight increases, the secondary radiation increases, but less rapidly. The 
ratio of secondary radiation to atomic weight is given in the fourth column of 
the table. 
The elements given in the above table fall into well-defined divisions in a 
striking manner if we plot a curve as in fig. 2 with atomic weight as abscissa 
and secondary radiation as ordinate. Carbon is in one division, and is the only 
member of that division given in the table; sodium, magnesium, aluminium, 
phosphorus, and sulphur form a second division ; potassium, calcium, chromium, 
iron, nickel, cobalt, copper, zinc, arsenic, and selenium form a third division ; 
molybdenum, silver, cadmium, tin, antimony, and iodine a fourth; tungsten, 
platinum, mercury, lead, and bismuth a fifth ; and uranium is in a sixth division. 
It will be observed that this method of division corresponds exactly to the 
division of the elements into “‘ periods” employed in chemistry. The divisions 
mentioned above correspond in order to the first and second short periods, the 
first, second, fourth, and fifth long periods. Our table contains no member of the 
third long period, the space for it occurring between iodine and tungsten. 
The elements contained in each chemical period lie on a part of the curve 
which is practically a straight line. Going from one period to the next there is 
very little increase of secondary radiation, so that there is a marked change in 
