Iv] NATURE OF THE RADIATIONS 137 
93. Connection between absorption and density. Since 
in all cases the radiations first diminish approximately according 
to an exponential law with the distance traversed, the intensity [ 
after passing through a thickness « is given by J = J,e-” where X 
is the absorption constant and J, the initial intensity. 
The following table shows the value of X with different radia- 
tions for air and aluminium. 
Radiation \ for aluminium for air 
Excited radiation ... 830 42 
Thorium ais Apes leny Dleedox0) 69 
Radium ae ae 1600 90 
Uranium ee nae 2750 16 
Taking the density of air at 20° C. and 760 mms. as 0:00120 
compared with water as unity, the following table shows the value 
of X divided by density for the different radiations. 
Radiation Aluminium Air 
Excited radiation ... 320 350 
Thorium) 9s Rie 480 550 
Radium Nes ae 620 740 
Uranium ... ae 1060 1300 
Comparing aluminium and air, the absorption is thus roughly 
proportional to the density for all the radiations. The divergence, 
however, between the absorption-density numbers is large when 
two metals lke tin and aluminium are compared. The value of X 
for tin is not much greater than for aluminium, although the 
density is nearly three times as great. 
If the absorption is proportional to the density, the absorption 
in a gas should vary directly as the pressure, and this is found to 
be the case. Some results on this subject have been given by the 
writer (loc. cit.) for uranium rays between pressures of 1/4 and 1 
atmosphere. Owens (loc. cit.) examined the absorption of the a 
radiation in air from thoria between the pressures of 0°5 to 3 
atmospheres and found that the absorption varied directly as the 
pressure. 
The variation of absorption with density for the projected 
positive particles is thus very similar to the law for the projected 
negative particles and for cathode rays. The absorption, in both 
cases, depends mainly on the density, but is not in all cases directly 
