134 



NA TURE 



[July 28, 1923 



investigations bearing directly upon experimental 



Sroblems. We have, therefore, arranged with Messrs. 

 •liver and Boyd, Edinburgh, to undertake the 

 publication of the British Journal of Experimental 

 Biology, the first number of wliich will appear in 

 September next. The new journal will receive 

 communications in comparative physiology, experi- 

 mental embryology, genetics, and animal behaviour, 

 as well as cytological, morphological, and histo- 

 logical contributions bearing on current experimental 

 problems. It wDl also publish by invitation authori- 

 tative r6sumds of recent progress in various fields of 

 inquiry. Any relevant original contribution will be 

 considered for publication. 



Inquiries may be addressed to the Animal Breeding 

 Research Department, the University, Edinburgh. 



F. A. E. Crew. 



W. J. Dakin. 



J. Heslop Harrison. 



Lancelot T. Hogben. 



J. Johnstone. 



F. H. A. Marshall. 



Guy C. Robson. 



A. M. Carr Saunders. 



T. McLean Thompson. 



An £^instein Paradox. 



The fallacy of the argument put forward by Prof. 

 R. W. Genese, in the former part of his letter in 

 Nature of June 2, p. 742, lies in his supposing that the 

 time / at which K sees the light-signal from L is related 

 to the time t', when K^ sees the same signal, by the 

 transformation 



t' = ^{t-vxlc^), 

 where ^ = {1 -v^jc^yK 



If we suppose the light-signal to be emitted from 

 L at a time T (in K's system) and Tj (in K^'s system), 

 then 



T^ = ^{T -vxlc^) (I) 



x^=^^{x-vT). .... (2) 



where x = KL, x^ = KjL, 



. Suppose now that K receives the signal at time t 

 (in his system) and that K^ receives it at time t^' (as 

 judged by Kj's system). Let t-^ be the time in K's 

 system corresponding to t^' in Kj's system. Then 



t = T + xlc (3) 



V = Ti + ^i/c, .... (4) 



and tj'=p{ti-vxjc^). . . . (5) 



Substitution from (i) and (2) in (4) gives, with (3), 



tj'=:lBi(l -VJC), 



and comparison with (5) shows that f+fj. 



A little careful consideration of these equations will 

 now show that the supposed paradox does not arise 

 for the case x^ = o. J. T. Combridge. 



King's College, Strand. 



Multiple Temperature Incubator. 



In the course of some experimental work on insects 

 which we have been carrying out, it was necessary 

 to have a large number of constant temperatures. 

 As it was impossible to have a complete incubator 

 for every temperature, an incubator was designed 

 by Mr. T. W. Kirkpatrick and myself to give a con- 

 tinuous series of constant temperatures. 



The principle used is the conduction of heat along 

 an insulated metal bar between two constant tempera- 



NO. 2804, VOL. I 12] 



tures. In practice one of these is an ice-box and the 

 other a hot water bath at any convenient temperature. 

 Between the two is a bar, tube, or trough of metal, 

 four to twelve feet long, which has holes bored at 

 close intervals throughout its length. Both copper 

 and aluminium have been used for the conductmg 

 bar. The whole is well insulated to avoid the 

 influence of the daily temperature change. 



The apparatus has exceeded our expectations and 

 would probably be of great use to investigators in 

 other fields. Full details with scale drawmgs and 

 temperature charts will be pubhshed shortly in a 

 Bulletin of the Ministry of Agriculture of Egypt, which 

 will be sent to any one who is interested. 



C. B. Williams. 



Ministry of Agriculture (Entomological Section), 

 Cairo, June 20. 



\ 



Phosphorescence caused by Active Nitrogen. 



In order to prepare aluminium chloride for atomic 

 weight determination, I burnt pure aluminium metal 

 in a current of pure dry chlorine. Before starting 

 the reaction, pure dry nitrogen was passed through 

 the apparatus to expel the air. After this has been 

 attained, the flow of nitrogen was stopped and a 

 slow current of pure dry chlorine was allowed to 

 pass over the metal. Since the pure dry gas reacts 

 very slowly with aluminium at ordinary room 

 temperature, the tube containing aluminium was 

 heated to about 500° C. After the completion of 

 the reaction, the aluminium chloride fomied and a 

 quantity of uncombined metal was cooled in a very 

 slow stream of nitrogen. As the red heat ceased, 

 a bright green phosphorescence appeared in the 

 reaction tube surrounding small pieces of corroded 

 uncombined metal. 



This phenomenon was excited the next day when 

 the synthesis was continued, and the leist traces of 

 chlorine were removed by nitrogen. In both cases 

 the afterglow disappeared after about one minute. 

 Two important facts should be added, namely : 



(i) The reaction tube — free of chlorine — with 

 aluminium chloride and the metal was heated again 

 to the same high temperature, and nitrogen was passed 

 over while the whole system was cooling down. The 

 bright green light did not appear. Nothing of tliis kind 

 of light was visible when the pure metal was heated 

 alone. This is a sufficient proof that the observed 

 afterglow in the former cases was not caused by a 

 trace of any known or unknown impurity of tlie 

 metal used. 



(2) The phenomenon was not observed during the 

 synthesis of aluminium bromide which Wcis carried 

 out by Prof. Th. W. Richards and me in the same 

 manner, and with an aluminium of the same origin. 



In Nature of May 5, p. 599, and May 26, p. 705, 

 were published letters by Prof. E. P. Lewis and 

 Mr. W. Jevons describing phosphorescence caused 

 by active nitrogen. These letters, particularly the 

 second, by Mr. W. Jevons, suggested to me that the 

 afterglow^ of aluminium left in the reaction tube was 

 very probably caused by active nitrogen. The 

 presence of traces of active nitrogen wsis caused 

 by the violent reaction of the chlorine left in the 

 tube with the aluminium metal. This reaction 

 activated some of the nitrogen passed over the 

 metal. When, however, all the chlorine was expelled 

 and the contents of the reaction tube were heated 

 as in the case described above, no phosphorescence 

 appeared. H. Krepelka. 



Department of Inorganic Chemistrj'-, 



Charles' University, 



Prague, Czechoslovakia. 



