AUGUST 8, 1931] NATURE 223

The conclusion that the gelatin functions differently stratified form will prove successful. \Ve also believe in the two cases does not appear to be justified. that just as bands of silver chloride in gelatine can be T. R. BOLAM. obtained by taking the former substance in sufficient Chemistry Department, The University, dilut.ion, the bands of the same substance in gelatine King's Buildings, Edinburgh, can be obtained even in slightly stronger solutions, May 5. provided the pH of gelatine is decreased sufficiently. 1 NATURE, Apri125, 1931, 127, 628. We have found experimentally that the inhibitive • Trans. Far. Soc., 22, 159 ; 1926. power of gelatine towards silver chromate can be ' Trans. Far. Soc., 22, 166, 167 ; 1926. altered considerably even by slight changes in its ' Trans. Far. Soc., 19, 2'l5 ; 1923. pH value. We are at present studying the effect of changing pH of gelatine on its efficiency in giving WITH reference to the letter by Mr. G. l\L Nabar banded precipitates (rings) of different sparingly and Dr. B. N. Desai in NATURE of April 25, p. 628, soluble substances. I should like to direct attention to the theory of All the points raised in the communications of banded precipitates that I published in various Drs. Bolam and Bradford are dealt with in detail in a journals as the result of a series of experiments from paper which will be sent shortly for publication. 1916 to 1922, and summarised in a paper on" Adsorp• B. N. DESAI. tive Stratification in Gels, V ". 1 In these papers the Physical Chemistry Laboratory, role of the ionic product and the metastable limit is Wilson College, Bombay, 7, July 3. fully discussed. 1 Trans. Far. Soc., 22, 159 ; 1926. The authors also appear to be under the impression ' Trans. Far. Soc., 19, 285 ; 1923. that silver chloride does not form Liesegang rings in gelatin ; but this is not quite correct. The pheno• menon is the same whatever the medium in which Oxidation Reduction Potential of Complex Iron the precipitate is formed; and as I have stated," No Compounds in Yeast. serious attempt to obtain a sufficiently insoluble WHEN an alkaline extract of yeast is saturated with precipitate in the stratified form has failed .... There ammonium sulphate there is precipitated, with the appears to be no evidence to show that stratification protein, cytochrome ' C ' and a complex iron com• will not occur, provided a precipitate can be obtained pound giving no visible spectrum. The latter can in a suitable state of subdivision from solutions be separated from the proteins slowly by ultra-filtra• of sufficient dilution." Actually the formation of tion. It remains in solution when the protein, with bands of Rilver chloride in dilute solution in gelatin the cytochrome ' C ', is precipitated by trichloracetic is recorded by W. M. Fischer,' bands of this substance acid. in water in capillary tubes were obtained hy Broder• This iron compound is of interest because its oxida• sen,3 and I obtained good bands in U-tubes in agar tion potential (E0 , referred to the normal hydrogen agar.4 The formation of banded precipitates of electrode,= +0·02 at pH 7·1) lies very close to that silver chloride, as with any other insoluble sub• at which the spectrum of cytochrome ' C ' appears stance, for example, silver chromate in agar agar, and disappears-a potential near which a variety of depends primarily on adjusting the supersaturation aerobic organisms appear to function. In quite low produced so that a suitably finely divided precipitate concentration it prevents the hydrogen electrode will be obtained. S. C. BRADFORD. from giving correct values in buffered solutions-a Science Museum, South Kensington, property which makes it seem likely that it can aet London, S.W.7. as a catalyst. 1 J. Soc. Chem. Ind., 7, 78; 1929. That the iron is complexly bound and not in the • Z . .Anoru. u . .Allg. Chem., 145, 311; 1925. form of simple ferrous or ferric ions is shown by the • Kolloid Z., 35, 21 ; 1924. ' Biochem. J., 10, 170 ; 1916. fact that even in a large excess of such a buffer as. a concentrated borate-tartrate mixture, the oxida• tion potential of this compound lies about BY the courtesy of the Editor of NATURE, I am 0·25 volt above that of ferric chloride added to the able to give the following reply to the comments of buffer. The iron of the compound does not react Drs. Bolam and Bradford on our letter on " Condition with sulphocyanide when oxidised, but does react of Silver Chromate in Gelatine Hydrolysed and slowly with aa dipyrridyl when reduced. Electro-Dialysed to Different Extents ", published An attempt to isolate the compound is now in in NA'I·URE of April 25, 1931, p. 628. progress. Because its oxidation potential is close to Drs. Bolam and Mackenzie 1 have not given any that of cytochrome ' C ', and because it is apt to ac• experiments in which silver chloride rings in gelatine company the latter in attempts to obtain the pure were obtained in the absence of silver chromate. pigment, there is some danger of confusing the effects In our experiments, the contact potential of the silver and properties of the two substances. electrode reached its final value as soon as the solu• .. . .. THOMAS B. CooLIDGE. tions of silver nitrate in gelatine arid potassium Converse Chemical Laboratory, chloride in gelatine were mixed, a result which does Cambridge, Mass., U.S.A., July 7. not agree with the observations of Langdon.2 Using the same sample of gelatine, it was, however, fmmd that with silver chromate the fall in the contact Measurement of Ultra-Violet Radiation. potential of the silver electrode was gradual. The WE are interested in the photographic method of contact potential measurements show that, in our recording the ultra-violet light of the sky, which has experiments, silver chloride in gelatine was not in been described by Dr. J. R. Ashworth.! ionic condition, but that silver chromate was. We With reference to the criticisms of Dr. Cunliffe 2 hold that, in the first case, gelatine acts as a protective the objection to the step-wedge does not appear to colloid, and in the second case, as an inhibiting agent; be important ; it is probable that the logarithmic and that it is the inhibitive power of gelat.ine which is ' law ' is approximately correct and in any case the responsible for the formation of Liesegang rings. wedge can be calibrated. It is, of course, well known We agree with Dr. Bradford that serious attempts that the photographic effect of an exposure is not a to obtain a sufficiently insoluble precipitate in the simple function of the time integral of the intensity No. 3223, VoL. 128] © 1931 Nature Publishing Group