A COMPARATIVE STUDY OF THE RADIOACTIVE 
MINERALS IN THE COLLECTION OF THE WAG- 
NER FREE INSTITUTE OF SCIENCE 
By CARL BOYER AND EDGAR T. WHERRY 
HAT the various minerals containing the elements uranium and thorium 
exhibit radioactive properties has been abundantly demonstrated by the 
researches of the Curies, Crookes, Pisani, Bardet, and others. Probably 
most of them have been studied with respect to their effect upon the electroscope, 
although their action on photographic plates has also attracted attention, and a 
few reproductions of the “radiographs” so obtained have been published. The 
lack of reports of attempts to compare directly the results obtained by the two 
methods, by employing the same specimens for both series of tests, suggested to 
Mr. Joseph Willcox the desirability of placing on record observations of this kind 
upon the specimens in the collection of the Wagner Free Institute of Science, 
and upon his initiative the work, which was carried out primarily with the idea 
of preparing a museum exhibit of the radioactive powers of these minerals, was 
undertaken by the writers. 
For the electroscopic measurements a small instrument of the type de- 
Signed by Professor Rutherford was employed. It consists of a cubical alu- 
minium box, 12 cm. on the side, within which are enclosed two brass disks 9 cm. 
in diameter. ‘These are placed in horizontal position 4 cm. apart, the lower one 
being held on a movable slide fitting a depression in the base of the instrument, 
and connected thereby with the ground, while the upper one is attached to a rod 
extending through an insulating plug of sulfur in the top of the box. A strip of 
copper at the top of this rod serves as a support for the leaflet, which in our ex- 
periments was of aluminium, and this is covered by a second somewhat smaller 
aluminium box, provided with opposite glass windows for observation, and with 
a charging hole in the top protected by a brass cap. After the instrument has 
been positively charged and all openings closed, the rate of leak is extremely low, 
the leaflet falling completely only after several days, so that no correction of the 
readings, extending as they do over but a few minutes, is necessary. Measure- 
ment of the rate of fall was made by a low-power microscope with micrometer 
eyepiece, the time required to pass over one division being observed with the aid 
of a stop-watch reading to fifths of seconds. 
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