FREE INSTITUTE OF SCIENCE 
A COMPARATIVE STUDY OF THE RADIOACTIVE MINERALS 
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The results are presented below in tabular form. The compositions given 
in the third column are simply averages for the mineral species and in no case 
have been determined on the specimens examined. It may be worth noting 
however, that monozite from North Carolina, the thorium content of which is 
well known to be considerably lower than that of the material here examined, 
gave results so faint that it has not been included in the list. The times given 
under electroscopic measurements are those required for the leaflet to fall over 
an arbitrary space, and are hence merely comparative. The activity figures 
are also entirely relative, being computed by calling the most active mineral 
unity, and dividing its time by that of each of the others; that is, A=15.2/T. 
The photographic activity has been obtained in a similar manner, by multiply 
ing the readings of the photometer by 0.2, 80 that the most active mineral again 
gave т. 
These figures make no pretense to accuracy, for successive readings of the 
electroscope varied over 5 per cent., and of the photometer as much as то per 
cent., but they are sufficiently definite to justify the conclusion that while in a 
general way the activity of a mineral measured by its effect upon the electroscope 
is proportional to that upon the photographic plate, and these are both roughly 
proportional to the uranium or thorium content, the thorium minerals are rela- 
tively less active photographically. While this is, of course, exactly what was 
to be expected from the results of the detailed studies of radioactive substances, 
according to which uranium gives out a much larger proportion of 8 rays than 
thorium, and it is these rays which produce the greater part of the action upon the 
sensitive plate, it can now be stated that it has been demonstrated by a direct 
comparison of the two methods on the same specimens of radioactive minerals. 
It also seemed worth while to publish a series of the radiographs produced 
by the various minerals, acting over different periods of time, since apparently 
no attempt has heretofore been made to figure such effects. The accompanying 
plates contain such as were intense enough to permit of satisfactory reproduction. 
These are not the ones used for the comparative measurements, for the majority 
of them were too faint, but have been made with solid specimens, the masks con 
sisting of sheet lead 1 mm. thick. These masks were impermeable to the radia 
tions in all but two cases. It will be observed that the uraninite from North 
Carolina, and more particularly the Ceylon thorianite, appear to have produced 
fainter effects in a week than in two days. Inspection of the one week’s expos 
ures shows, however, that in them the points of the stars are considerably 
