The extended equation can be made to fit the-observed isotherms to aw = 0·7, and, at the very point where divergence appears, the collagen hydrochlorides begin to exhibit abnormal properties which are best explained by postulating the presence of liquid water. Conclusion Water vapour adsorption isotherms at 25° have been measured for several modifications of silk and for hydrochlorides of silk and collagen. The curves have been analysed by the method of Brunauer, Emmett and Teller, and the derived constants have been shown to support the hypothesis of adsorption on polar groups of the protein molecule. The power of basic groups and of aliphatic hydroxyl to co-ordinate water molecules is unaffected by reaction with hydrogen chloride. The higher hydrochlorides of collagen undergo irreversible shrinkage and gelatinisation when the relative aqueous vapour pressure exceeds 0·7. At the same time their electrical conductivity increases rapidly with aw. These changes have been shown to support the explanation already advanced concerning the nature of collagen hydrochlorides. They also confirm the hypothesis of capillary condensation of water in the region of moderately high aw where the B.E.T. equation becomes inadequate. The author wishes to thank Professor F. G. Soper for permission to use the facilities of his laboratory, and the Royal Society of New Zealand for a grant from their Research Fund. References Brunauer, S., Emmett, P. H., and Teller, E., 1938. Adsorption of Gases in Multimolecular Layers. J. Amer. Chem. Soc., vol. 60, p. 309. Bull, H. B., 1944. Adsorption of Water Vapor by Proteins. Ibid., vol. 66, p. 1499. Cohn, E. J., and Edsall, J. T., 1943. Proteins, Aminoacids and Peptides, p. 358. Reinhold Publishing Corp., New York. Fischer, E., and Raske, K., 1907. Verwandlung des 1-Serins in d-Alanin. Berichte, vol. 40, p. 3717. Green, R. W., 1948. The Adsorption of Water Vapour on Collagen and Elastin. Trans. Roy. Soc. N.Z., vol. 77, p. 24. —— 1949. The Adsorption of Water Vapour on Casein. Ibid., vol. 77, p. 313. —— 1950. The Reaction of Hydrogen Chloride with Dry Proteins, Part 1. Ibid., vol. 78, p. 291. —— 1951. The Reaction of Hydrogen Chloride with Dry Proteins, Part 2. Ibid., vol. 79, p. 485. McLaughlin, G. D., and Theis, E. R., 1945a. The Chemistry of Leather Manufacture, p. 109. Reinhod Publishing Corp., New York. —— 1945b. Ibid., p. 125. Robinson, R. A., and Sinclair, D. A., 1934. The Activity Coefficients of the Alkali Chlorides and of Lithium Iodide in Aqueous Solution from Water Vapour Pressure Measurements. J. Amer. Chem. Soc., vol. 56, p. 1830. Tristram, G. R., 1949. Amino-acid Composition of Purified Proteins. In Anson, M. L., and Edsall, J. T., Advances in Protein Chemistry, vol. 5, p. 142.