September 22, 1923] 



NA TURE 



439 



near the earth, would be responsible for the fact 

 that the readjustment of the earth's crust after a 

 large portion had been removed in the course of the 

 formation of the moon was mainly effected in an 

 east and west direction. 



In these circumstances the excessive meridional 

 folding which Prof. Chamberlin postulates as a 

 lecessary result of tidal retardation could not be 

 looked for. 



The chief value of the formulation of a speculative 

 "hypothesis such as I have sketched out is in illustrat- 

 ing and emphasising the interest and importance of 

 detailed study of geological structures, region by 

 region and period by period, throughout the world. 

 It will only be when we have all the facts before us, 

 that we shall be able to solve with any assurance 

 jthe problems presented by the present configuration 

 )f the surface of the globe. 



With regard to the use of the words " rift " and 

 " rift- valley," the latter was originally and properly 

 used by Prof. J. W. Gregory for a structural valley 

 due essentially to tension, and I use " rift " in the 

 corresponding sense — of a split in the earth's crust 

 due likewise to tension. This is in close accordance 

 with the popular and literary use of the word " rift." 

 Should at any future time it be clearly proved that 

 the " great rift valley " was never in the whole 

 course of its existence associated with east and west 

 tension, it would then, I submit, have no longer a 

 right to the title. John W. Evans. 



Imperial College of Science and Technology, 

 South Kensington, S.W.7, 

 August 31. 



Stereoisomerism among Derivatives of Diphenyl. 



The cases of isomerism so far recorded among 

 derivatives of diphenyl, whether connected with 

 optical activity or not (Kenner and co-workers. Trans. 

 Chem. Soc, 1922, 121, 614, etc.), are interesting from 

 the point of view of the possibility of the existence 

 of a stable para - bond in benzene and, more 

 particularly, in diphenyl derivatives. Thus, any 

 2 : 2'-derivative of diphenyl should be capable of 

 optical activity on the basis of the general formula : 



.CH=CRv 



CH- 



-C— C 



CH 



\ch=ch/ \ch=ch/ 



which reveals the presence of four asymmetric carbon 

 atoms. 



From the same point of view, the isomeric dinitro- 

 benzidines (cf. for example, Brady and McHugh, 

 Trans. Chem. Soc, 1923, 123, 2047), and some of the 

 substances derived from them, also contain four 

 asymmetric carbon atoms, although this type does 

 not include cases of optically active substances at 

 present. 



The above suggestion opens up a large field for 

 investigation. It is interesting to note, in passing, 

 that diphenyl forms a tetra-ozonide, which may 

 indicate that the para-bonded condition is favoured 

 even by the parent hydroczirbon. E, E. Turner. 



East London College, 

 Mile End Road, E.i, 

 September 4. 



The Liesegang Phenomenon — an Historical Note. 



The discovery of the phenomenon of periodic 

 stratification in gels is attributed to Liesegang {Phot. 

 Archiv, 1896, 221). Historical accounts of earlier 



experimental work on the formation of precipitates of 

 sparingly soluble substances in gels are given by Wo. 

 Ostwald (" Grundriss der Kolloidchemie," Dresden, 

 1909, 208) and by Bradford {Biochemical Journal, 29, 

 29, 1920). The latter author states {loc. cit., p. 29) : 

 " The first observation of a series of layers (produced 

 by periodic precipitation in gels) must be ascribed 

 to Lupton (Nature, 47, 13 (1892))." It may be 

 observed that Ord published experiments before 

 this date on the formation of calcium oxalate in 

 isinglass gels. Details of these experiments are given 

 in his book (" The influence of Colloids upon Crys- 

 talline Form and Cohesion," London, 1879), which, 

 in the writer's opinion, has not received the attention 

 it deserves. It appears from the passage quoted 

 below {op. cit., p. 108) that Ord before 1879 (the 

 actual date of the experiment is given in the text as 

 March 12, 1869) had obtamed stratified precipitates 

 of calcium oxalate : 



" The deposit (of calcium oxalate) was not uniform, 

 but somewhat stratified, forming a layer of greatest 

 density near the calcium solution, a layer of less 

 density, with some opalescence, near the oxalic 

 solution, and several intermediate layers of still less 

 density, with alternate spaces of extreme scantiness of 

 deposit." 



In the light of these facts it seems that the priority 

 of the discovery should be taken by Ord. 



J. R. I. Hepburn. 



119 Richmond Road, London, N.i, 

 August 18. 



NO. 2812, VOL. I 12] 



Urease as a Product of Bacterium radicicola. 



The letter by Prof. Werner in Nature of August 

 II " On the Presence of Urease in the Nodules of the 

 Roots of the Leguminous Plants," induces me to 

 state that urease is also produced by the pure 

 cultures of Bacterium radicicola, and much more pro- 

 fusely than by the nodules. Such forms as Vicics, 

 Trifolii, Pisi, are particularly strong in this respect, 

 while Ornithopodis and Lupini are but feeble urease- 

 producers. 



It is interesting to observe that urease is also, in 

 certain cases, a product of the normal papilionaceous 

 plants, first discovered by Takeuchi in the beans 

 of Soja hispida, and by me in the seeds and the rind 

 of the branches of Cytisus Laburnum and Glycine 

 chinensis. 



The simplest way for the demonstration of the 

 enzyme is the plate-method which I have described 

 in Centralblatt f. Bakteriologie, 2te Abt., Bd. 5, p. 

 323, 1893, and Archives N^erlandaises, 1895. As, 

 however, B. radicicola does not grow well on broth- 

 gelatin, or yeast-decoct-gelatin with ij per cent, 

 urea, the detection of the enzyme must be made 

 with material taken from colonies previously grown 

 on peas-Ieaf-gelatin, with 2 per cent, cane-sugar, and 

 then used as little lumps, placed on the yeast-decoct- 

 urea-gelatin plate. After a few minutes the beautiful 

 " iris-phenomenon " becomes visible if urease is 

 present, as a consequence of the production of 

 ammonium-carbonate which precipitates the calcium- 

 carbonate and calcium-phosphate in the particular 

 manner proper to this experiment. The addition of 

 some calcium-malate to the yeast-urea-gelatin en- 

 hances the sensibility of the iris-reaction. 



The discovery of urease in B. radicicola was the 

 result of experiments on the nutrition of this bac- 

 terium, performed in 1919 and 1920, with the co- 

 operation of Mr. Ir. L. E. den Dooren de Jong at 

 Delft. M. W. Beijerinck. 



Gorssel, Holland. 



