4:12 



Sir W. Crookes. 



: 7. Roughly speaking, the action of pitchblende is in proportion to 

 the percentage of uranium in the mineral. Finely powdered pitch- 

 blende of different degrees of richness were experimented with. Cells 

 were made of thick lead pipes half an inch internal diameter and one 

 inch long, closed at the lower ends with card. They were filled with 

 powdered pitchblende, one containing 43 per cent. UrsOs and the 

 other 12 per cent. UrsOs. A sensitive plate being covered with black 

 paper, the lead cells were laid on it and kept in total darkness for 

 120 hours. The intensity of the spot under the 43 per cent, ore on 

 development was found to be at least three times that of the one under 

 the 12 per cent. ore. 



Two lead cells were taken, one being a quarter of an inch long and 

 the other two inches long. They were filled completely with the 43 

 per cent, ore, and a sensitive plate exposed to their action for forty- 

 eight hours. On developing it was doubtful whether any difference 

 existed in the intensity of the two spots, proving that the action does 

 not pass through much thickness of active material, a quarter of an 

 inch being equal in effect to two inches. 



No difference in the action was noticed when the bottom of the cell 

 was made of thin glass cemented on, instead of card. 



Four cells were filled with pitchblende and placed side by side on a 

 sensitive plate. ; After having acted twenty-four hours the first was 

 removed, the second after forty-eight hours, the third after seventj^- 

 two hours, and the last was kept on for ninety-six hours. On develop- 

 ing the plate the spots had intensities varying with the lengths of 

 exposure, and in about the right proportion, on the assumption that 

 double the time of action gives double the intensity of blackening. 



8. For convenience of comparison I had a number of glass cells 

 made, three-quarters of an inch wide and deep, so that they could 

 either be sealed up or closed with a cork. Apiece of ^apparatus was 

 made so. as to take radiographs of samples with more ease and cer- 

 tainty. A lead plate, 2 mm. thick, 6J inches long, and 2^ inches wide, 

 has circular holes punched in it, one inch in diameter. Under the 

 thick plate of lead is; another thinner plate, made of pure assay foil, 

 and having holes in it, concentric with the others, but barely f inch 

 in diameter, so that one of the small cells will not pass through the 

 lower hole, but will pass easily through the upper hole, and thus be 

 kept in place. To prevent contact between the lead plate and the 

 sensitive surface a thin sheet of celluloid is fixed beneath, with holes 

 punched in it concentric with those in the lead plates. In the top 

 left corner of the lead plates is a short steel pin, which can be pressed 

 on the sensitive plate and so register its position in respect to the cells 

 experimented with. The lead and celluloid plates are then bound 

 together and the whole is fitted into a shallow wooden tray with a 

 light-tight cover. 



