438 
SIR W. CROOKES ON ACQUIRED RADIO-ACTIVITY. 
face was just removed, the adjoining corner thus being freshly exposed diamond 
crystal. It was put on a sensitive film in such a position that it could be subsequently 
examined and compared with the developed image, so that the active portions of the 
surfaces, natural and ground off, could be seen. It was allowed to act for five days, 
when the film was developed. The result indicated very decidedly that where the 
surface of the diamond had been removed it was no longer radio-active (13). 
Experiments with a very Active Diamond Crystal. 
17. A fine crystal of diamond from Kimberley which had been kept in a bottle of 
radium bromide for some months was tested on a sensitive film and in the electroscope. 
It was found to be highly active, and was set aside for further experiments. It 
was luminous in the dark, quickly discharged the electroscope, and caused an 
inactive diamond held near it to phosphoresce. It was put on the surface of a 
screen coated with small crystals of barium platinocyanide, and caused scintillations 
the same as on a blende screen, but feebler (ll, 26, 27, 30, 33). 
18. A blende screen was made by coating a glass slip with very sensitive zinc 
sulphide. This was laid on the diamond, the ZwS side next the diamond; the 
scintillations were easily seen through it in all their characteristic appearance, with 
concentration at the edges and corners. A piece of aluminium sheet, 0'06 mm. 
thick, was moved about between the screen and crystal of diamond, and there was 
no doubt whatever that the aluminium stopped all the scintillations. It was 
absolutely dark where the aluminium covered the crystal. 
19. The diamond (17) was then held in contact with the card back of a platino¬ 
cyanide of barium screen. The luminous patch due to (3- or y-rays, or to both, 
was quite evident, moving about as the diamond was moved. The sheet of 
aluminium foil, used in the former experiment (O'06 mm. thick) was put between 
the crystal and the back of the screen, and there was little, if any, diminution in 
the luminosity. It is therefore quite certain that the radio-active diamond gives 
off other rays beside a-rays, and that the rays can penetrate aluminium 0'06 mm. 
thick and the card back of the screen. 
20. The diamond (17) was put into various fluorescent solutions (uranine in 
water, Silberrard’s p-nitroso-dimethylanilinenaphthaline compound, and quinine sul¬ 
phate). It did not occasion the slightest fluorescence, although its own faint 
luminosity could be seen in the liquid. 
21. The active diamond (17) was cemented to a plate of glass, and six small 
pillars of lead cemented round it the same height as the stone. The whole was 
inverted on a sensitive film, and kept in the dark for 2'25 hours. Another 
experiment was then tried with the same apparatus, only altering the position 
of the crystal so that the lead pillars were opposite the angles of the crystal. The 
exposure in this case was 6'5 hours. The pictures on development showed a strong 
