The principal acid produced is lactic; the acetic acid is produced directly from the glucoses by the above-mentioned bacteria without any preliminary alcoholic fermentation by yeasts.
The mode of action of the drench on the skins may be summed up as follows:--
1. The solution of the last traces of lime which has not been removed by the bate, by the organic acids produced by the fermentation, and the subsequent swelling action of these on the skin fibres. The acids also dissolve a small amount of skin substance.
2. Simultaneously with (1) the distension and floating of the skins by gases produced by the fermentation, so enabling them better to take up the acids.
3. The mechanical absorption of dirt by the particles of bran or flour in the drench.
It is not proposed to enter into further detail, since the next chapter, consisting of the original papers of Dr. W. H. Willcox and the author, contains a pretty complete account of the analysis of the drench and theory of the drenching process.
1. FURTHER CONTRIBUTION ON THE NATURE OF BRAN FERMENTATION. By J. T. Wood & W. H. Willcox, B.Sc.(Lond.).
(Reprinted from the Journal of the Society of Chemical Industry, May 31, 1893. No. 5, vol. xii. p. 422.)
The paper which one of us had the honour of reading before this Section on December 11, 1890,[161] dealt chiefly with methods of bacteriological research, but especially in connection with the fermentation of bran as applied in the manufacture of light leathers. The object of the present paper is to give an account of further research into the nature of the fermentation and its products, the former communication being very incomplete. In further investigating the matter, we endeavoured--
1. To obtain a complete knowledge of the products of the actual fermentation as it takes place in practice.
2. To discover in what way the ferment acts, both on the materials fermenting and on the skins.
3. To examine in the same way the products of a pure cultivation of the bacterium causing the fermentation.
1. *The Products of the Actual Fermentation.*--These may be divided into three groups:--(1) gases; (2) volatile bodies; (3) non-volatile bodies. It was stated in the former paper that the ferment produced an inflammable gas along with considerable quantities of CO_{ 2}, H_{ 2}S, etc. The inflammable gas was thought by analogy from the researches of Tappeiner[162] to be methane; it has, however, proved to be pure hydrogen. The absence of hydro-carbons was shown by the following method. Some of the gases were collected, the CO_{ 2} and SH_{ 2} removed by absorption with KOH, and the remaining gases exploded in a eudiometer tube with oxygen. The gases which remained after explosion did not diminish in volume after standing over KOH solution, showing absence of the paraffins and olefines.