of Radium, and Secondary Rays which they produce. 643 
repeated, and so on. The measurements for determining 
the radiation from a substance in terms of that from a 
standard substance were carried out in the following 
manner : — Two readings were taken without any radiator. 
Two readings were then taken with the radiator under in- 
vestigation placed in position. Lastly, two readings were 
taken with the standard radiator. This process was repeated 
at least seven or eight times. The mean of the first set of 
readings was then substracted from each mean of the two 
other sets of readings, the result giving the relative secondary 
radiation from the substance under investigation and the 
standard radiator. 
The following numbers will give an idea of the relative 
magnitude of the leaks measured. A deflexion of 1800 
scale-divisions in 4 minutes was obtained when the screen 
was of iron and "6 cm. thick, and no radiator was used. 
Deflexions of the same amount were obtained with a lead 
radiator 1*8 mm. thick in 3 minutes, and with a zinc radiator 
3 mm. thick in 2 minutes 8 seconds. An electrometer of 
the Dolezalek type, fitted up with a telescope and scale, was 
used in these experiments. Since the leaks were all very 
small the experiments were very tedious, the determination 
of the amount of secondary radiation from a substance in 
terms of that from a standard substance requiring about 
three hours. 
Table I. contains the results of some measurements carried 
out in the manner described. The first vertical column gives 
the nature of the radiators and their thickness. The first 
horizontal column gives the nature and thickness of the 
screens used. The numbers in each vertical column give 
the relative amounts of secondary radiation from the sub- 
stances given in the first column, when the screen given at 
the top of the column was used. The radiation from lead, 
the standard substance, has been put equal to 100 for each 
screen. The relative radiations obtained with the same 
screen depend, it will be observed, on the selective partial 
absorption of the radiations by the screen, and the nature 
of the secondary radiation emitted by each substance, and 
on the relative masses of the substances used. Since some of 
the radiation generated in a substance is absorbed by the 
substance, the nature of the radiation emitted by a given 
mass of substance must depend somewhat on its shape and 
size. The shape of each radiator was therefore made the 
same as that of the standard substance with which it was 
compared. The radiation from mercury was compared with 
that from lead by placing a quantity of mercury in a thin 
