204 Hacxert— The Secondary Radiation excited by y Rays. 
consistent with this condition, in order to obtain numbers easily observed. ‘To 
carry out this method, a lead stopper, in shape like the frustrum of a cone, was 
cast to fit perfectly into and fill completely the conical opening. It was 5 cm. in 
height, and was found to absorb 95 per cent. of the y rays which fell on it. This 
was used to separate out the ionisation produced by the radiation from the plate 
from the total ionisation in the cylinder. This total ionisation was due to several 
causes :—(1) the natural ionisation ; (2) the rays which have not passed through 
the conical opening, but which have reached the cylinder directly through the 
lead screening, and the secondary effects due to these rays; (3) the secondary 
radiation produced by the y rays which pass through the conical opening, and 
fall on the exposed plate of the substance under examination. ‘The insertion of 
the lead stopper in the conical opening leaves the first two unchanged, and 
reduces the third to 5 per cent. of its former value. The difference in the 
readings before and after the lead stopper is inserted is therefore the secondary 
radiation produced in the exposed plate by the 95 per cent. of the y rays 
absorbed by the stopper. 
The essential condition in this method is that the horizontal plate should be 
large enough to shield the cylinder completely from all parts of the conical 
opening, so that no secondary rays produced at the edges of the conical opening 
can reach the cylinder, but are completely absorbed by the plate. If this is so, 
the difference in the two readings can only be due to the change in the radiation 
from the plate caused by the absorption of the y rays producing it. ‘There is no. 
difficulty in getting large plates of the common elements. The general properties 
of the secondary radiation were therefore investigated with large plates of lead, 
copper, and aluminium, using a large opening to get a strong pencil of y rays. 
But, in making a general comparison of the radiating powers of the elements, a 
smaller opening, with a lead stopper corresponding, was made in order to make use 
of a collection of metal plates 5 cm. sq., brought together by Professor McClelland 
for his researches on B rays. When plates of the elements were not obtainable, 
powders in trays of the same size were used instead, as no sensible difference 
between the metal and its powder could be observed in those cases where both 
were available. It may be added that the plates and layers of powder must be 
thick enough to absorb completely any 8 radiation falling on their under-surface, 
coming from the sides of the opening below. 
The secondary radiation excited by the y rays comes from a small depth 
below the surface. The intensity of the exciting rays may then be taken as the 
intensity of the y rays after they have passed through the plate. If each plate 
were used singly, the radiation would vary, both from the change in the 
substance, and in the intensity of the exciting rays. This difficulty is easily 
avoided by allowing the y rays to pass through two plates by placing one over 
