144 Mr. S. J. Allan on the 
active was partly dissolved in ammonia and the solution 
evaporated, the residue gave the same rate of decay as the 
excited activity on the solid copper. The activity excited 
ona piece of rubber also gave the same rate of decay. 
It has been shown by C. T. R. Wilson,* and independently 
by the author +, that freshly fallen snow, when evaporated 
down to dryness, leaves behind a residue which is temporarily 
radioactive. The rate of decay of this radioactive residue 
was examined. Snow was gathered from a thin sheet on the 
surface during a snow storm, evaporated down to dryness in 
a shallow dish, and placed in the parallel plate apparatus 
described above. <A litre of snow, when evaporated, produces 
about the same effect as one-fifth of a gramme of uranium. 
The rate of decay was found to follow an exponential law, and 
the activity fell to half value in from 30 to 32 minutes. Thus 
there is a distinct difference between the rate of decay of the 
radioactivity on snow and that obtained from the air. The 
penetrating power, however, was found to be about the 
same for both. In fig. 3 are two curves showing the rate of 
decay of the radioactivity from snow. 
C. T. R. Wilson has shown that freshly fallen rain, when 
evaporated, leaves a radioactive residue. This residue was 
tested in the same way as that obtained from snow, and it 
was found that the rate of decay was the same, the activity 
fell to half value in about 32 minutes. Wilson states that 
the activity of the rain-water he examined fell to one-quarter 
value in one hour. In fig. 4 are shown two curves for the 
decay of the radioactivity from rain-water, taken on different 
days. 
Fie. 5 shows the curves of decay of the radioactivity from 
snow and of the activity excited on a lead wire and of the 
excited radioactivity from a copper wire transferred to felt, 
all plotted to the same scale for the sake of comparison. 
Absorption of the Excited Activity by Solids. 
A number of experiments were made with various solids to 
see if the absorption-density law held for the excited activity. 
The excited activity on a copper wire was transferred to a 
thin piece of leather moistened with ammonia, and the leather 
was placed between the parallel plates of the testing apparatus. 
Readings were taken when the leather was bare and when 
covered with thin layers of various solids. From the curve 
of decay and these readings the percentage absorption could 
* Proc. Camb. Phil. Soc. 1902, No. 11, p. 428. 
+ American Physical Society, Jau. 1903; Phys. Rey, May 1908. 

