483 
1908-9.] A Simple Radioscope and a Radiometer. 
creased beyond what is just necessary to bring down a few of the 
natural nuclei, then we get more drops on the micrometer with the same 
radioactivity, but there does not seem to be more due to the radioactivity ; 
the increase seems to be due to the greater number of the natural nuclei 
brought down. 
The method generally adopted was to make the compression necessary 
to give the odd drops, and then bring to a fixed point some radioactive 
substance which is kept as a standard test and note the number of drops. 
Repeat the test with slightly varying compressions, but always with the 
standard radioactive body, and note results. It will be found that the 
most satisfactory tests are made when the number of drops on the natural 
nuclei is small, and in that condition practically all the drops are due to the 
radioactivity. If higher compressions are used, we have to deduct from 
the total number of drops those due to the natural nuclei. If at any time 
there is any suspicion that the test is not correct owing to some imperfec- 
tion in the apparatus, all we have to do is to present the standard radio- 
active substance and see if we get the correct number of drops. 
For the measurement of strong radioactive substances we must either 
interpose screens or put them at a distance from the instrument, because 
the number of drops given by them is far too great to be counted. 
Reference has already been made to the importance of the quickness 
of the expansion in these condensation experiments, and the question 
naturally arose, Was the arrangement used the best for the purpose ? or 
would some other form of release not give better results ? Some experi- 
ments were accordingly made with other forms of release which promised 
to make the expansion quicker. The apparatus shown in fig. 4 was tried ; 
this was interposed between the compression chamber and the compressor, 
taking the place of R, fig. 3. V is a tube open at both ends and provided 
with a branch tube on which is a stopcock. The end of the tube C r is 
connected with C, fig. 2, by means of a small piece of india-rubber tube, 
while the branch tube C" is connected with the compressor. S is a thick 
piece of sheet india-rubber which closes the end of the tube Y, the india- 
rubber being fixed on the lever T, which is hinged at its lower end, and 
held at its upper end by the catch U. The lever requires considerable 
force to compress the india-rubber and engage with U. The object of this 
is to make the india-rubber act as a spring as well as an air-tight valve 
to the end of the tube Y, so that when the lever is released from the 
catch U it will be forced rapidly away, so opening the end of Y quickly, 
and thus giving the air in the compression chamber a free outlet. After 
the compression has been made, the stopcock on the tube C" is closed, and 
