of Radium, and Secondary Rays which they produce. 657 
relative a moan is of radiation of a number of substances. 
The first two columns in the table give the nature and mass 
of the radiator, and the third column gives the values of the 
secondary radiation. The fourth column gives the radiation 
per unit mass calculated on the assumption that the amount 
of radiation absorbed by each radiator is proportional to its 
mass. The values for carbon and lead are those obtained 
with the cardboard screen, while the values for the substances 
of Group II. are the mean of the values given in Table II. ; 
they differ little from those obtained with the cardboard 
screen. The values of the radiation per c.c. of the substances 
of Group II. only in the table can be compared with one 
another, as just explained. They give approximately the 
relative amounts of radiation of Groups II. and III. produced 
per c.c. in each substance. It will be seen that the values do 
not differ much from one another. This is a result that 
would be expected if the mass of an atom is proportional to 
the number of electrons it contains. The values obtained 
would probabh' be more nearly equal to one another if the 
masses of the radiators used had been equal to one another. 
The difference in the calculated amount of radiation per unit 
mass for lead and a substance of Group II. cannot be 
altogether due to the selective absorption of the cardboard 
screen, or to a difference in the absorption of the radiated 
rays. It seems to indicate, therefore, that the radiation of 
Groups II. and III. produced in a unit mass of lead is less 
than the radiation of Groups I. and III. produced in unit 
mass of a substance of Group II. This is probably due to 
the amount of radiation of Group I. being greater than that 
of either Group II. or Group III. We obtained some evi- 
dence of this, it will be remembered, when discussing the 
number of different primary and secondary groups of rays. 
We will now discuss at some length the transformation of 
primary rays into secondary. When a beam of primary 
y rays passes through a plate, it generates secondary rays in 
the plate corresponding to the different groups of primary 
rays : the amount of radiation in a secondary group of rays 
depending on the absorption of the corresponding primary 
o-roup. Since one of the primary groups of rays will be 
better absorbed by the plate than either of the remaining 
groups, the amount of secondary radiation generated corre- 
sponding to this group must be larger than that corresponding 
to either of the remaining groups. The amount and nature 
of the secondary radiation which is not absorbed by the plate, 
and which is therefore radiated away from the plate, depends 
on the selective absorption of the different groups of rays by 
