477 
1908-9.] A Simple Radioscope and a Radiometer. 
than A, because it entirely depends on the expansion of the air. As this 
form of instrument requires to he frequently removed from its stand and 
turned upside down for the purpose of wetting the interior walls, and 
possesses no advantages, its use was abandoned. This form of compression 
chamber required a pressure of 20 cm. to give the first or rainy stage of 
condensation, and 30 cm. to give the foggy stage. The reason for it 
requiring a greater compression than Wilson’s, is due to the resistance 
offered to free expansion by the tube C and the ball D. 
The arrangement shown at R, fig. 1, was also tried. In this case the tube 
is similar to A, but the expansion is stopped when the air has expanded to 
its original volume ; that is, the movement of the water is stopped before 
expanding its energy in expanding the air. This was done by the arrange- 
ment shown in R. A floating valve rose and fell with the water in the right 
arm of the U tube, so that, after compression and release, when the water 
rose to its original level the valve suddenly closed the outlet. As no advantage 
was observed in its action, it was not further experimented with. The 
reason for making this instrument was that, theoretically, the best conditions 
of the expansions in these condensation experiments would appear to be 
that the expansion be done as quickly as possible. During the first part of 
the expansion quickness does not matter so much, because no condensation 
is taking place on the nuclei, but the quicker the expansion is made after a 
certain stage is reached the more nuclei will become active. Now, the 
arrangement shown at A, one would imagine, would rather tend to make the 
expansion slow towards the end, and it was thought that by stopping it 
when at its maximum rate that an advantage might be derived — giving it, of 
course, the same amount of expansion as before ; but, so far as the purpose 
for which the instrument is intended, no advantage was observed. This 
instrument required a compression of 21 '5 cm. to give the rainy stage and 
32 cm. to give the foggy condition, but the pressure required was variable 
according to the setting. If the valve did not float just at the level of its 
seat — that is, floated slightly below it — then, as might be expected, the pres- 
sure required was less, as the moving water is allowed to do a little expansion. 
When working with the expansion chamber as shown at A, fig. 1, the 
water after expansion rushes back into the chamber B and by splashing 
develops a few nuclei, but these are so few they can hardly be said to 
interfere with the results ; and this splashing has the great practical 
advantage of keeping the inside walls of the tube wet. 
Returning now to the apparatus shown in fig. 1, suppose the com- 
pression is adjusted so as to give only an odd drop or two falling in the 
chamber B. If while in this condition any radioactive substance is brought 
