January 29, 1904.] 



SCIENCE. 



195 



without regard to the movements of the larger 

 organization. They might all meet together 

 at times and places where the accommodations 

 were adequate, "but such places would be few 

 and far between. 



Of course, this would result, under the pres- 

 ent conditions, in a society and a section with 

 similar aims meeting in different places at the 

 same time and a member of both might have 

 difficulty in deciding which of the two he 

 should attend. But the remedy is a simple 

 one. These separate societies have, by right 

 of preemption, a claim upon the Christmas 

 holidays for their meetings. The whole 

 trouble has been caused by the American Asso- 

 ciation for the Advancement of Science, which 

 has encroached upon this period and is now 

 trying to force the independent organizations 

 to accommodate themselves to its actions. All 

 that is necessary for full harmony is that the 

 association return to its summer meetings, 

 leaving the Christmas vacation free to its 

 rightful possessors. J. S. Kingsley. 



TiTFTS College, 

 January 13, 1904. 



THE SCI^^TTLLATIONS OP RADIUM. 



The phenomenon of the scintillation of a 

 phosphorescent screen, under the influence of 

 the radium bombardment, which was first de- 

 scribed by Sir William Orookes, is one of the 

 most impressive spectacles which we have had 

 for a long time. 



As comparatively few of us have had an 

 opportunity of witnessing this remarkable 

 sight, I have prepared about two dozen ' spin- 

 thariscopes,' which I shall be very glad to pass 

 around among my colleagues, on the condition 

 that they be promptly returned. 



Last autumn, while experimenting with 

 some phosphorescent materials, I fotmd that 

 the scintillations could be as easily seen when 

 the radivim was mixed with the phosphorescent 

 powder (the mixture being pressed between 

 two plates of glass) as in the usual form of 

 Orookes's spinthariscope. 



If one sits for several minutes in an ahso- 

 lutely dark room, and then examines the plate 

 with a powerful pocket magnifying glass, the 

 appearance reminds one of an enormous star 

 cluster as seen in a telescope, the individual 



stars lighting up and disappearing in rapid 

 succession, producing an impression which has 

 been likened to that produced by moonlight 

 on rippling water. 



Whether the flashes are produced by the im- 

 pact of the individual electrons which consti- 

 tute the a rays, as was imagined by Crookes, 

 or whether they represent microscopic cleav- 

 ages which are occurring in the crystals as a 

 result of the bombardment, as Becquerel be- 

 lieves, is' still an open question. The fact 

 that hundreds of flashes appear every few 

 seconds, the action showing no signs of abate- 

 ment after several months, makes it difficult 

 to believe that each flash represents a split in 

 a crystal, unless one is prepared to accept the 

 doctrine of ' infinity divisibility.' It is, per- 

 haps, equally hard to believe that the impact 

 of a single electron is responsible for each 

 flash. The obvious way of settling this ques- 

 tion would be to make a rough estimate of the 

 number of flashes produced in a given time 

 by a very small amount of radium of very 

 low activity, and see if the number was of the 

 same order of magnitude as the number of 

 positive electrons given off in the same time. 

 If the number of emitted electrons far exceeds 

 the number of flashes, we may find a way out 

 of the difficulty by assuming that the electrons 

 are thrown out in intermittent streams, the 

 impact of each ' squirt ' producing a flash. 



On carefully scrutinizing the screen it is 

 almost impossible to avoid forming the opinion 

 that the points of light are in motion, the 

 whole field squirming with light, like a colony 

 of infusoria under the microscope. This ap- 

 pearance is, perhaps, a little more pronounced 

 with the Orookes spinthariscope, in which a 

 speck of highly active radium is mounted at 

 a little distance above the screen. If this 

 motion should turn out to be real and not 

 illusory it could, perhaps, be explained by a 

 slight sweeping motion of the streams of 

 electrons emitted by the radium. Such specu- 

 lations are scarcely worth while, however, in 

 view of the very deceptive nature of illusions 

 of motion. The plates which I have prepared 

 for distribution are packed in small tin boxes, 

 which can be sealed up in an ordinary envelope. 

 Institutions desiring to borrow one will be 



