Page:Radio-active substances.djvu/23

Radio-active Minerals.

I have examined many minerals in my apparatus; certain of them gave evidence of radio-activity, e.g., pitchblende, thorite, orangite, fergusonite, cleveite, chalcolite, autunite, monazite, &c. The following is a table giving in amperes the intensity, $i$, of the current obtained with metallic uranium and with different minerals:—

	$i\times 10^{11.}$
Uranium........................	2·3
Pitchblende from Johanngeorgenstadt......	8·3
""Joachimsthal.........	7·0
""Pzibran.........	6·5
""Cornwallis.........	1·6
Cleveite........................	1·4
Chalcolite........................	5·2
Autunite........................	2·7
Various thorites..................$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$	0·1 0·3 0·7 1·3 1·4
Orangite........................	2·0
Monazite........................	0·5
Xenotime........................	0·03
Æschynite........................	0·7
Fergusonite (two samples)............$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	0·4 0·1
Samarskite........................	1·1
Niobite (two samples)...............$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	0·1 0·3
Tantalite........................	0·02
Carnotite........................	6·2

The current obtained with orangite (native oxide of thorium) varied greatly with the thickness of the layer. By increasing this thickness from 0·25 m.m. to 6 m.m. the current increased from 1·8 to 2·3.

All the minerals which showed radio-activity contained uranium or thorium: their activity is therefore not surprising, but the intensity of the action in certain cases is unexpected. Thus pitchblendes (ores of uranium oxide) are found which are four times as active as metallic uranium. Chalcolite (double phosphate of copper and uranium) is twice as active as uranium. Autunite (phosphate of uranium and calcium) is as active as uranium. These facts do not accord with previous conclusions, according