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Proceedings of the Royal Society of Edinburgh. [Sess. 
cause the nuclei to grow to dust size, as a less expansion gives condensation 
with incandescent light than with gas flame ; and if the air was exposed to 
the incandescent light for fifteen minutes, it gave condensation on very 
slight expansion, just like dust, while the strongest radium salt I possessed 
gave no indication of being able to increase the size of the nuclei. 
Some tests were also made with methylated alcohol in the U tube, but 
beyond the lower expansions required to cause condensation on the ions, no 
advantage was observed. 
Another form of instrument was tried which seems to have some 
advantages over the U tube owing to the expansion being quicker and 
more suddenly stopped. In this instrument the compression chamber is a 
short length of an ordinary test tube, connected directly with a pear-shaped 
india-rubber compression ball. The test tube was connected with the ball 
by means of an ordinary f-inch brass union joint, one end of the union 
joint being cemented into the neck of the ball while the test tube was 
cemented into the other end. This joint is necessary for cleaning the tube 
and filling it with water. The compressing apparatus had to be modified 
to meet the new conditions. The compressor shown in fig. 1 is turned 
upside down, and has an opening made in the base-board large enough for 
the passage of the test tube and union joint, while the hinged board in this 
case presses on the bottom of the compression ball. By this arrangement 
the compression chamber projects upwards through the compressor, and the 
compressions and expansions are made in the same manner as in the other 
instrument. 
The Radiometer. 
The principal defect of the radioscope above described is that it only 
gives a rough indication of the strengths of the radioactive substances 
brought near it. It, however, seemed possible, if we were to adopt the 
principle already in use in the dust-counter, to make an instrument that 
would give numerical values of the strengths of the different radioactive 
substances, as by this arrangement we would be able to count the number 
of drops formed in the test chamber, and thus obtain definite figures of the 
strengths. To carry out this idea, the instrument shown in fig. 2 was 
constructed. B is the condensation chamber, in which the air is compressed 
and expanded. This chamber is made of brass, and has an opening E in 
the top and another opening F in the bottom. These openings are closed 
with glass plates, the upper opening being covered with clear glass, and 
the lower one with a glass micrometer ruled with cross lines 1 mm. apart. 
The chamber B is connected with the compression apparatus shown in 
