176 MecCiettanp—On Secondary Radiation. 
and the secondary radiation is by no means proportional to the density ; platinum, 
for example, produces less radiation than lead, and tin more than copper. 
It gives a more interesting result to compare the secondary radiation with the 
atomic weight of the substance; the atomic weight and the ratio of the secondary 
radiation to the atomic weight are given in the table. It will be seen that throughout 
the table, the greater the atomic weight the greater is the secondary radiation. 
The ratio is not constant, but it will be noticed that the substances fall into 
groups with respect to this ratio. There is a well-marked group for which the 
ratio is unity, and another for which the ratio is one-half. Of course the absolute 
value of the ratio is merely a question of the scale in which the secondary 
radiation is expressed. 
It is intended to extend largely the list of substances examined to see if the 
dependence of the radiation on the atomic weight holds throughout, and to follow 
out the apparent grouping. . 
The radio-active substances—radium, thorium, and uranium—are all substances 
of high atomic weight ; and the results now before us indicate that the greater the 
atomic weight the greater is the secondary radiation under the action of f rays. 
The secondary radiation seems therefore to be closely allied to the spontaneous 
radiation from radio-active substances. The impact of the 8 rays produces a dis- 
turbance sufficient to cause the disintegration of the atom in substances which, in 
the normal state, are in stable equilibrium. 
The dependence of the secondary radiation on the atomic weight, and the close 
relation between it and radio-activity, suggest that the activity of radium might 
be increased by the action of its own radiations, and Voller (Physikalische 
Zeitschrift, Dec. 1, 1904) has obtained indications of such an action. We must 
remember, however, that the secondary radiation we are considering is one of B 
particles; so that, in extending our table to radium, we should consider a 
transformation product of radium which emits 6 particles. 
It is interesting to compare the table given above with the results of 
observers who have studied the small radio-activity of ordinary substances when 
there is no artificially applied exciting cause. McLennan found that lead was 
about twice as active as zinc, while tin gave an intermediate value; this is the 
same order as in our table of substances. A comparison with numbers given by 
Strutt for the activity of a number of substances does not, however, show much 
agreement. 
In the above comparison of different substances, the secondary rays passed 
through a single sheet of tinfoil -013 mm. thick; another set of observations was 
taken with the end of the tube 7’ covered with a sheet of aluminium ‘3 mm thick, 
in addition to the tinfoil. No difference in the order of the substances took 
place; but if we again express the secondary radiation from lead as 100, the 
