JL:ly, I921.1 CURRENT TOPICS. I43

Phenosafranin in P h o t o g r a p h y . - - G r e a t interest has been awakened by the discovery of the s t rong densit izing powers of the dyestuff known commercial ly as phenosafranin in Europe and supplied by an American manufac turer under the title Safranin A. extra. Dr.. Liippo-Cramer, Technical Director of Kranseder & Co.. Munich, whose fame in photographic research is interna- tional, called a t tent ion to the peculiar p roper ty that dilute solu- tions of this color have in diminishing the sensibility of exposed plates wi thout appreciably affecting the latent image. As might be expected, many other substances have similar properties, but none so far examined equals phenosafranin. Solutions of not over one part of the color to 2ooo of water are efficient, and such solu- tion keeps. Lf ippo-Cramer has fur ther found that the dye mark- edly accelerates the action of hydroquinone, and that it also acts as preservat ive of developers. A solution of the material has re- cently been put on the market by an English firm under the title " Desensitol ." This is a very concentra ted solution, and is directed to be diluted to abont fifty times for use. A. and L. Lmniere and Sevewitz have recently presented a communicat ion "~o the French Pfmtographic Society, detail ing the results of many exper iments to determine what other substances are densitizers. The paper has appeared in very full abst ract in a recent issue of La l~c~'z~e Fr~znfaise dc Photographic. Lfil~p~-Cramer has published a small book giving an account of his researches in the field, and discussing some of the theories of the action.

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Note on the Possibility of Separating Mercury into Its Iso- topic Forms by Centrifuging. J. H. J. POOLE. (Phil. Mag., May, I 92 I . ) - - Ac c o r d ing to Aston's results mercury is a mixture of six isotopes having atomic weights of ~97, 198, I99, 200, 202 and 2o 4. To simplify the problem of finding on theoretical grounds whether it be possible to separate the liquid into its isotopes by centrifuging several assumptions are m a d e . Only two isotopes are considered at present, and these have a difference in atomic weight of 4 units. T h e y are in approximate ly equal amounts and are supposed to differ only in mass ; all other constants being the same for both. Mercury is regarded as incompressible.

T h e conditions of equil ibrium for such a mixture in a rota t ing tube placed in a centr ifugal field of force are examined, and it is found that the difference of density between the two ends of the ro ta t ing tube would be only one part in 3o,ooo for 9ooo revolut ions per minute. So minute a difference could hardly be detected. It is suggested that a centr i fuge could be made to run at 6o,ooo revolu- tions per minute, and that with this appara tus the difference of density would reach z ~ parts per thousand, which could be de- t e c t e d . Af te r~a t ta in ing this conclusion the author puts a curb on anticit~ati0n by saying: " The results obtained would apparent ly