428 Prof. Wood on the 



I refrained from publishing the results, in the hope and 

 belief that some one else would succeed in solving the 

 question in a more conclusive manner. As no further work 

 appears to have been done, and no more satisfactory theory 

 has been advanced, I have decided to publish them for what 

 they are worth, together with the conclusion which I have 

 drawn from them. 



It was hoped that the cleavage hypothesis could be either 

 strengthened or weakened by determining the duration of the 

 spinthariscope flashes, and the flashes produced by cleavage. 

 If the order of magnitude was found to be different in the 

 two cases, it would be a very strong argument against the 

 hypothesis of Becquerel. The duration of the flash due to 

 the radium bombardment was determined without difficulty. 

 The rim of a wooden disk, mounted on the shaft of a small 

 electric motor, was coated with zinc sulphide, and a speck of 

 radium supported above it on a needle-point. The bombarded 

 surface was viewed in a dark room with a small lens. The 

 flashes remained sharp even when the motor was running at 

 a pretty good rate, but on further increasing the speed they 

 became less distinct, being drawn out into short streaks. It 

 was pretty definitely determined that the duration of the 

 flash was somewhere between ^- Q and ^o °^ a secon d. 



The only method I was able to devise for determining the 

 duration of the triboluminescence was the simple expedient 

 of touching the rim of the revolving disk with the point of a 

 slender rod of glass. The o-low due to the crushing of the 

 crystals extended halfway around the disk, even when it was 

 running at low speed. In addition to the long streak of 

 light, there is always a bright star at the point where the 

 rod is in contact with the sulphide surface This is 

 probably due to the fact that some of the luminous crystals 

 are held back, by the rod, stick to it in other words, and does 

 not necessarily indicate that the first flash is of very brief 

 duration. If the latter were the case, we should expect the 

 glowing star to be slightly drawn out at very high speeds, 

 which is not the case. This experiment appears to indicate 

 that the flashes due to the radium bombardment have a much 

 shorter duration than the flashes which result from fractures 

 of the crystals, and there seems to be no way of escaping 

 from the conclusion that the two phenomena are not very 

 closely related. 



A photographic study of the phenomenon was then under- 

 taken to determine the integral effect of a large number of 

 flashes, for it seemed possible that the flashes might recur at 

 certain points more often than at others. 



