472 SECTIONAL TRANSACTIONS.—B, C. 
small non-tan molecules is followed by the absorption of tannins possessing 
large molecules. Reasons are given for upholding the view that the com- 
bination of collagen with tannins is mainly electrostatic in character, being 
partly salt formation between tannin and protein and partly association 
arising from the polar character of the molecules. 
Mr. F. C. THompson.—The gelatin-tannin reaction. 
This interesting reaction has a long history beginning with Seguin and 
Humphrey Davy, but has not yet been completely elucidated in spite of 
the efforts of many workers. The variability in the composition of the 
precipitate under differing experimental conditions attracted the attention 
of the earliest investigators and led ultimately to the somewhat vague 
characterisation of the reaction as a ‘ colloidal’ or ‘ adsorption’ reaction. 
Attempts to gain a closer insight into the mechanism have been made in 
several quarters. J.T. Wood showed that the reaction had little in common 
with what is usually considered to be adsorption. Stiasny has emphasised 
the importance of the degree of dispersion of the reactants, whilst Kruyt 
has considered the reaction as an electrical discharge followed by dehydration. 
A simple chemical theory assuming the formation of an insoluble salt from 
the tannin as acid and the gelatin as base (a salt which largely resists 
hydrolysis by reason of its high degree of insolubility) is fairly satisfactory 
up to a point but does not cover all the facts of the case. An adequate 
chemical theory will probably have to take account, following Freudenberg, 
of the molecular compounds formed by phenols and organic bases. 
Dr. F. E. Humpureys.—Factors influencing the tanning properties 
of tan liquors and extracts. 
The average molecular weight and degree of hydration of the constituents 
of the more common vegetable tanning materials and extracts have been 
determined. ‘The influence of these factors on the tanning properties of 
the materials examined is discussed. 
AFTERNOON. 
Excursion to Fort Dunlop. 
SECTION C.—GEOLOGY. 
Friday, September 1—Wednesday, September 6. 
GEOLOGICAL ExcuRSION TO SHROPSHIRE AND THE WELSH 
BORDERLAND. 
This excursion, carried out under the general direction of Prof. W. W. 
Watts, F.R.S., occupied the time from September 1 till the opening of 
the meeting. The headquarters were at Much Wenlock, and the districts 
visited included the following : the Wrekin, Charlton, and Overley areas 
under the guidance of Dr. R. W. Pocock ; the Cambrian and Longmyn- 
dian Rocks of Caer Caradoc and Church Stretton under the guidance of 
