378 SECTIONAL TRANSACTIONS.—B, 
Wednesday, August 13. 
14. Joint Discussion with Section A on Colloids. 
(a) Prof. J. W. McBain, F.R.S.—(1) The Conception and Pro- 
perties of the Electrical Double Layer and its Relation to Ionic 
Migration. (2) The Rate of Saponification of Oils and Fats by 
Alkali. (8) Limitations to the Use of Indicators in Alkaline 
Solutions and in the Presence of Soaps. (4) The States of Matter 
exemplified by Soaps and their Solutions. 
(1) The Conception and Properties of the Electrical Double Layer 
and its Relation to Ionic Migration. 
Our present knowledge of the double layer between liquids and such materials 
as glass, membranes, and colloidal particles is reviewed, and the numerous and 
conflicting assumptions which have been put forward in calculating the ‘ contact 
potential’ are stated explicitly. It is emphasised that the contact potentials 
currently employed have a fictitious significance, and it is therefore urged 
that experimental results be recorded in the form in which they are observed— 
namely, movement in an electric field or electromotive force set up by bodily 
movement. 
A conception of the double layer with sparsely distributed mobile ions is 
developed which appears to harmonise the existing data, including also those 
for absolute electrode potentials and for the behaviour of sols, gels, and curds 
of soap (cf. earlier paper by Miss Laing). If this conception is a true inter- 
pretation of the existing data, the fundamental assumptions made in all pre- 
vious mathematical treatment of this subject are invalid. 
(2) The Rate of Saponification of Oils aud Fats by Alkali. 
It has been found possible to measure the rate of saponification of oils by 
means of a hydrogen electrode at 90°, and to isolate the various factors involved. 
The rate depends,-for example, upon the amount of emulsified oil, and it is found 
that even with vigorous stirring only a limited proportion of the oil present 
is truly emulsified. Excellent monomolecular constants are obtained for the, — 
dependence of the rate on the hydroxyl ion present. 
(3) Limitations to the Use of Indicators in Alkaline Solutions and 
in the Presence of Soaps. 
Most of the commercial indicators have been tested at 18° and 90° in solu- 
tions of pure sodium hydroxide, and serious discrepancies are found in results 
with Sorensen’s standard buffer solutions and with pure hydroxide, most of 
the indicators giving quite misleading and useless results. Two, or possibly 
three, indicators can be used in the presence of soaps, and under suitable condi- 
tions confirm measurements of the hydrolysis of soaps obtained by other methods. 
(4) The States of Matter exemplified by Soaps and their Solutions. 
Soaps form an almost unique material for the precise investigation of the 
relations between a number of interesting states of matter. Any one aqueous 
soap can be prepared in the form of true hexagonal crystal lamelle : white, 
opaque (crystalline) curd fibres, transparent liquid crystals (conic anisotropic 
liquids), and transparent isotropic solutions which are dark when examined 
with crossed Nicols. The isotropic solutions are crystalloids or colloidal elec- 
trolytes, and the latter are either fluids or true jellies depending in a most 
regular manner upon temperature and concentration. A mass of quantitative 
Sights 
data is now available for each of these states, and some insight has been obtained ~ 
into their constitution and relationships. 
(b) Prof. Wiper D. Bancrort.—The Permeability of Membranes. 
(c) Prof. H. S. 'Tavnor.—The Adsorption from Silver Salt Solu- 
tions by Silver Iodide. 
