NEARLY all the organic substances exhibiting gel formation in water are compounds of complex structure and high molecular weight. In order to elucidate the mechanism of gel formation, in recent years search has been made for substances of relatively low molecular weight and simple constitution, and the dyestuffs , benzo-purpurin, chrysophenone, thionin blue, the soaps, the organic sulphonic acids and camphorylphenylthiosemicarbazide have been used. As Gortner and Hoffman have recently pointed out [1921], benzoylcystine is the simplest known type of organic substance which exhibits gelation with water and it therefore appeared to present a suitable material with which to study some of the elementary properties of gels in greater detail. The preparation of dibenzoylcystine used in this work was essentially that of Brenzinger [1892] and Gortner and Hoffman [1921], except that the substance, after recrystallisation from dilute alcohol, was extracted for several hours with benzene in a Soxhlet apparatus to remove completely all traces of benzoic acid and benzoyl chloride. The melting-point of our product was 1890 (uncorrected) which is some eight degrees higher than that given by the older authors.

In a recent paper, one of us with Harris [Wolf and Harris, 1917] has shown that, in the fermentation with Bacillus Welchii (B. perfringens), all activity ceases when the reaction of the medium reaches a certain hydrogen ion concentration. The cessation of growth appeared to be connected with the production of acids. We were able to confirm the statements of previous observers, who have worked on acid production, that normal butyric acid was one of the constituents of the fermentationi liquid, by isolating the acid in a fairly pure condition and identifying it. Other acids were also present. With the B. sporogenes of Metchnikoff, there was also a large amount of acid formed when the organisms acted on various media. In milk it was difficult to say from what component of the medium the acid arose.

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A report to the Medical Research Council.- (Received September 5th, 1923.) ONE of the factors which may be anticipated to affect the growth and metabolism of bacteria is surface tension.The relation of this physical factor to other biological problems such as muscle action or the deposition of salts in plant cells [Macallum, 1911] has been the subject of much study; the relation to bacterial growth has received little attention.If, however, one considers the results obtained with surface films by J. J. Thomson [1888] and others it is evident that the increased concentration of substances, such as peptone, at the air/liquid interface ought to have a pronounced effect on those organisms growing at the surface. This should especially be the case with micro-organisms like C. diphtheriae where experience has shown that efficient toxin production is associated with good pellicle formation.In a study of certain factors affecting toxin production my attention was drawn to this aspect of the subject by the statements of practically every -bacteriologist who has worked in this field as to the necessity of removing all but inevitable traces of fat from the medium before it is used. The fact that pellicle is also essential would appear to be connected with the particular conditions of the air/liquid interface.In a paper published in 1919 Larson, Cantwell and Hartzell [1919] made some very interesting statements regarding the effect of lowering the surface tension of media on the growth and habit of bacteria. The protocols of their experiments are unfortunately lacking in details of the methods used for determining surface tension. On lowering the surface tension of their media they observed that anaerobic bacteria grew in the presence of air, that organisms, usually forming pellicles, grew throughout the liquid and did not form a film and that spore-bearing organisms no longer produced spores. Evidently the unusual environment of the bacteria completely changed their habit of growth.