486 Proceedings of the Koyal Society of Edinburgh. [Sess. 
arrangement screwed on to the wire W. The substance being firmly secured, 
the union joint is replaced and screwed up, then a few compressions and 
expansions are made to throw down any dust clinging to the sample, after 
which the wire is pushed forward into the apparatus till the forceps is just 
visible on the edge of the field seen through the lens. Compression and 
expansion are now made and result noted. It may be mentioned that a 
piece of pitchblende about the size of a pin-head gives copious showers 
under these conditions. Its action, however, does not seem to be constant, 
as the shower may sometimes permit of counting the drops, while the next 
shower will be so dense it is impossible to do so. Salts of radium, however, 
seem to give showers of constant density. For instance, a small disc of 
brass about 2 mm. diameter was wetted with a very weak solution of a 
radium salt and allowed to dry. The solution used for this purpose was 
very weak, being made of a small speck of a very weak salt dissolved in, 
half an ounce of water. On testing this small quantity of radium inside 
the chamber it gave very dense showers about 50 drops per sq. mm., while 
through the aluminium window it only gave about 1 per mm. It may be 
mentioned that this disc did not give a scintillation of action when tested 
with the phosphorescent screen. 
A similar disc of brass was put into a bottle in which was a weak 
radium salt to test the effects of the emanations from the radium. After 
the disc had been in the bottle five hours to collect radium A — the active 
deposit of the emanations — it was put inside the expansion chamber and 
gave at first about 30 drops per sq. mm. Another disc put in the 
same bottle with radium for one hour gave nearly the same number. In 
both cases the number remained high for some time, but in a few hours 
both ceased to act. As the experiments were made simply for testing 
the radiometer, no notes were taken of the rates of decay. In collecting 
the radioactive deposit care was taken that the disc did not touch the 
bottle containing the radium. The disc was fitted to a new cork which had 
been fitted to the bottle, and so arranged that the cork supported the disc 
in the middle of the space over the radium, and all metal parts connected 
with the disc were further cleaned with emery cloth to remove any chance 
of contamination. 
In using this method of working great care has to be exercised that the 
body introduced into the expansion chamber does not interfere with the 
expansion of the air. As already pointed out, any alteration in the tube 
between the expansion chamber and the compressor alters the results. In 
making these experiments it was found necessary to test the forceps to see 
if their introduction did not interfere with the results, when it was found 
