266 NATURE MARCH 3, 1945, Vol
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266 NATURE MARCH 3, 1945, VoL. 155 LETTERS TO THE EDITORS the same time most tenaciously and strongly ad sorbed• to a polar surface is now explainable by our The Editors do not hold themselves responsible new observations, and the stratochemical layering. for opinions expressed by their correspondents. The electrical behaviour of the protein-coated silica No notice is taken of anonymous communications. particles, etc., is due not to the primary mono-layer, but to the secondary reversible layer. It should be noted that the arrangement required 'Sensitizing' by Protective Colloids (of polar-nonpolar alternance, or 'amphipathy', to THIS is the name given 1 to the following use Hartley's term) seems consistent with Astbury's phenomenon. It was observed2 that the addition X-ray diffraction conclusion that "polar and non of a quite small amount of a hydrophile colloid to a polar side chains follow one another alternately sol of a hydrophobe colloid caused the latter to be along the main chains" in the protein structure10• coagulated (precipitated) by a much smaller amount The mechanism of 'sensitizing' evidently· derives (at much lower concentration) of a precipitating from the amphipathic character of the protective electrolyte than in total absence of the protective colloid, which is manifest not only in the proteins colloid. However, suitable larger amounts of the but aJso in such polyoses as the starches, dextrins, latter made necessary larger amounts of precipitating gums and pectins, and in kindred bodies, such as 1 electrolyte than in the absence of the hydrophile polyvinyl alcohoP • Some part of the initial precipit colloid, hence the term 'protective colloid' for the ating effect of these hydrophile colloids can be re latter. placed by an alternate electrolyte--and vice versa. In a letter in Nature in 1921" I suggested that many In all cases, the primary monolayer furnishes lipoid of the properties of the so-called (at that time) surfaces, coherence of which is coagulation. This is 'emulsoid colloids', and particularly of proteins, de followed by repeptization by addition of further rived from a duplex character of their molecules in hydrophile colloid'. respect of their possessing both hydrophile polar and With my colleagues I shall deal with the subject hydrophobe but organophile non-polar atom-groups. more fully in subsequent publications. The conception was developed more explicitly in a s. E . SHEPPARD. monograph on gelatin • to provide a basic strata Chemistry Department, chemical architecture for the micelles of proteins•. Research Laboratories, Experimental evidence was presented from the Eastman Kodak Company, effect of gelatin on the interfacial tension between R ochester, N.Y. water and toluene and from the of gelatin Dec. 19. at this interface out of dilute solutions•. 'l!'reundlich, H., "Kapillarchemie" (Auf!. 2, Leipzig, 1922), 799 Recent investigations by me and my collaborators7 a seq. Also for non-colloidal non-electrolytes, idem., 636 et seq. have shown that the adsorption of many basic dyes 'Henri, V., et al., C.R. Acad. Sci., 55, 1671 (1003). to silver halides occurs in two stages. In the primary 'Sheppard, S. E., Nature, 107, 73 (1921). 'Sheppard, S. E., "Gelatin in Photography". I. Monographs on the -and priming-=--stage, the dye m olecules are de Theory of Photography, No. 3. (New York: D. Van Nostrand posited in a mono-layer, with the polar or ventral Company, 1923), 188. (to suggest an anatomical image) aspect of the 'cf. also bul. and Eng. Chem., 13, 37 (1921). 'Sheppard, S. E., and Sweet, S. S., J. Am<lr. Chem. Soc., 44 , 2797 molecule (ion) adhering to the silver halide surface, (1922). and the hydrophobe (or dorsal) edge projecting into 7 Sheppard, S. E ., Atti. X Congress Internat. di Cl!im., VI, 234 (1938); the aqueous solution. After saturation is effected, also Sheppard, S. E. , Lambcrt,R. H.,and Walker , R. D.,J. Chern. Phys., 7, 265 (1939). a second mono-layer begins to build on. This does • Abramson , H. A., "Electrokinetic Phenomena" (Amer. Chern. Soc. not form a rigid but a reversible layer, of the Lang Monog.), 1934. muir type, because now the non-polar hydrophobe 'cf. Sheppard, S. E., Lambert, R. H. , and Keenan, R. L., J. Amer. aspects of the dye molecules are attracted each to Chern. Soc., 36, 174 (1932). 10 Astbury, W., J . Chem. Soc., 337 (1942). each between solution and primary adsorbate, while 11 cf. Sheppard, S. E., and Newsome, P. T., J. Chem. Phys., also Pt. 2 the polar, hydrophile groups of the secondary layer (in the press, ibid). are attached to the water. There are complications attendant on the forma tion of dimerized and polymerized aggregates of the dye molecules which need n ot detain us here. Age of the Saline Series in the Punjab Salt What is important is that we now have evidence from direct adsorption measurements from dilute Range solution, as well as from experiments on the coagula PROF. B. SARNI's important observations1 have tion and repeptization of silver halide sols by dilute necessitated a reconsideration of this problem. In gelatin solutions, that a quite similar process to that order to review the geological evidence on the ground, with dyes occurs in the adsorption of proteins to the an excursion was arranged to examine several sections silver halides and other little-soluble polar solids. which had led E. R. Gee, of the Geological Survey of The electrophoretic and kindred investigations of India, and other geologists to the conclusion that the H. A. Abramson 8 and many others have led to the Saline Series of the Salt Range is · of Cambrian or general conclusion that : "The fact that not only the pre-Cambrian age. Prof. Sahni was unfortunately isoelectric points but also the electric mobilities of unable to take part in the excursion, the party con quartz particles covered with serum albumin or sisting of the undersigned. egg albumin [or gelatin] are very nearly identical The sections visited were near Khewra in the eastern with the values of mobility for the respective dis part of the range, and in the Warcha-Sakesar area solved protein indicates that practically all the polar of the middle western portion. This examination groups of the protein molecules are available even showed that an Eocene or later age for the Saline after adsorption has occurred". The apparent paradox Series is irreconcilable with the field evidence, which, whereby the polar force fields of the protein should in our opinion, indicates that this series is Cambrian be quite free even when the molecules are at one and or older. © 1945 Nature Publishing Group.