276 Scientific Intelligence. 



radium. One day's exposure to a few milligrams of radium pro- 

 duces a distinct effect in restoring the color. At the same time 

 the property of thermoluminescence is acquired. Moreover Strutt 

 showed, in the year 1906, that zircon is a distinctly radioactive 

 mineral, containing hundreds of times as much radioactive matter 

 as ordinary rock masses. He therefore advances the hypothesis 

 that " the zircons found in lavas have had their colour and 

 thermoluminescence restored by the slow action, during prolonged 

 ages, of the radium they themselves contain.'''' Unfortunately the 

 color of fresh crystals is saturated, that is, further exposure to 

 radioactive matter does not deepen it. Consequently, it is not 

 possible to determine the age of hyacinths from the rate of colora- 

 tion and the amount of radioactive material present. Experi- 

 ments were performed to test this point and it was found that the 

 original saturated tint was imparted to a specimen which had 

 been decolorized by heating and then exposed for four days to 

 the radiations from a radium salt. 



On the other hand, the opaque brown zircons, as originally 

 found, are not thermoluminescent and they cannot be made so 

 by exposure to radium. Furthermore they do not lose their color 

 when moderately heated. Since there are reasons for believing 

 that hyacinths are formed from these opaque zircons b}^ the action 

 of a bath of molten basalt, an attempt was made to imitate this 

 process experimentally. Basalt was kept melted in a platinum 

 crucible over a gas furnace and some opaque zircons were 

 immersed in the molten mass for 24 hours. After this they were 

 extracted and found to be quite white, though not transparent. 

 On exposure to radium they assumed the redder tint of hyacinths 

 and became thermoluminescent. In conclusion Strutt says: "The 

 only outstanding point is the transparency of natural hyacinths." 

 "This may result in some way from the gently increasing, very 

 prolonged action of the molten basalt under geological conditions, 

 which cannot be artifically imitated." — Proc. Roy. Soc, vol. 

 lxxxix (A), p. 405. ii. s. u. 



9. Sup]?ressio?i of the Photoelectric Effect. — A great step in 

 advance seems to have been made by the theoretical considera- 

 tions and experimental skill of K. Fredenhagen and Hans 

 Kctstner. For lack of space a description of the apparatus used 

 and an account of the extraordinary refinement of the method 

 will not be given. The experimental data refer only to zinc, 

 since the investigation of other metallic surfaces is still in 

 progress. The results obtained may be summarized as follows : 



(1) No measurable photoelectric effect can be obtained with pure 

 zinc surfaces when all traces of reactive gases have been removed. 

 That is, the photoelectric effect is less than one thousandth part 

 of the value found under ordinary experimental conditions. 



(2) The " fatigue " which occurs in the presence of reactive gases 

 can be accounted for by the reaction with adsorbed gases. The 

 " recovery " is explained by the diffusion of reactive gases towards 

 the zinc, whereby the velocity of the reaction and consequently 



