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.

H . L .

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

I44 CURRENT TOPICS. [J. F. I.

hardly justify the expense entailed in constructing the special centrifuge." The analysis of elements into isotypes is based on the study of positive ray phenomena, which a re not well known to chemists. If it be possible to" verify the analysis by appeal to the familiar mechanical method of centrifuging, it is to be hoped that some means may be discovered for constructing the needed apparatus.

The author holds that, if liquid neon could be substituted for mercury, a difference in density ought to be obtained, since the two isotopes differ by Io per cent. The difficulty of running at the temperature of the liquid would not be small.

G. F. S.

The Spectrum of Helium in the Extreme Ultra-Violet. H. FRICKE and THEODORE LYMAN. (Phil. Mag., May, I92I .)--Hitherto investigations of the spectrum of helium in the Schumann region have been made of necessity by the use of a strong disruptive dis- charge, which introduced impurities into the gas through its action on the walls of the tube and on the electrodes. This state of affairs led to uncertainty in the origin to which the observed lines were to be attributed. Though other lines were believed by one investigator or another to be due to helium, yet in the range of wave-length from I7oo to 600 angstrom units only two lines could with assurance be assigned to helium. Within the last two years it has been found that there is a resonance potential for helium corresponding to a wave-length of about 600 units.

The gas was at a pressure of .8 millimetre of mercury. The current employed was direct and varied from 20 to 40 milli- amperes. The vacuum spectroscope was smaller than its prede- cessor, the grating having a radius of only 20 cm. The change in dimensions materially reduced the absorption of light by gas. The helium as originally prepared was pure, but by the time it was transferred to the apparatus it sometimes had gained traces of hydrogen and of oxides of carbon. Under the new conditions of ex.periment the strongest line of all was that occurring at 585 umts. Its authenticity was confirmed by experiments with the previously employed, larger spectroscope, with which the record of the line was faint but unmistakable. It is interesting to note that this line, whose existence is known by experiment, fits into the speculative scheme of Bohr and Sommerfeld.

G. F. S.

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