46 



prolonged period. This slow outward diffusion from solid solution 

 is the counterpart of the phenomenon observed when the initial talcing 

 up of the gas is followed by a period of very slow absorption. The 

 same behaviour was observed by Lefebure^^ (1914) for the rate at 

 which gases are taken up by celluloid, and a similar explanation is 

 given. The comparatively slow saturation of beer contauiing high 

 proportion of soUds is, without doubt, to be interpreted as evidence 

 of solution of the gas in the disperse phase, the rate of which is 

 directly proj)ortional to the rate of diffusion. 



PracticaUy nothing is known of the mechanism of this solution 

 in the soUd phase, and it is instructive to compare the phenomena 

 observed by Findlay and King" with the results obtained by 

 Anderson^ who investigated the rate of elimination of water vapour 

 from silicic acid gels^^. 



Anderson showed that the rate depends upon the capillary structure 

 of the gel and towards the end of the dehydration the rate is influenced 

 by the same phenomena which is responsible for a section of the 

 Van Bemmelen^i curve. Anderson's results recalculated in terms of 

 the velocity coefficient used by Findlay and Eong yield a curve of 

 the same form as that obtained by the latter for starch, dextrin, and 

 suspensions of charcoal. The influence in the Van Bemmelen and 

 Anderson curve is considered to indicate soUd solution of the gas 

 in the walls of the capfllaries (Zsigmondy^s). The slow evolution 

 of gas from solutions of starch, peptone, and dextrin is obviously of 

 considerable importance in connection with the palatabiUty and 

 sparMing quaUty of beverages, and if, as it appears, this depends 

 upon the slow diffusion outward from the disperse phase, every 

 faciUty must be afforded, during carbonation, for the gas to dissolve 

 in that phase. In fact the beer should, as Langer^" has shown by 

 practical tests, contain a high proportion of residual extracts, and 

 be carbonated at low temperatui-e. The assumption by Siegfried^^ 

 of the formation of a carbamic acid which at the higher temperatm'e 

 of the palate gives free carbon dioxide would seem to be mmecessary. 

 The formation of head is governed by the rate of effervescence, the 

 increase being greater, the greater the concentration of colloids, within 

 Hmits obviously determined by viscosity. More important, however, 

 is the absence of substances tending to reduce the surface tension 

 (Bau^), and according to Ihnen" under similar conditions those beers 

 rich in dextrins retain the foam best. 



When the comphcated phenomena of gas solubflity in colloidal 

 solutions is better understood, it will no doubt be fomad that much 

 of the conflicting evidence as to the effect of coUoids wiU be explained, 

 simply, as due to alteration in the physical condition of the particular 

 colloid. It has long been known that gelatine solutions if repeatedly 

 heated above 60° C. lose their power of jeUying when cooled. The 

 experiments of Menz^', Ganett-S von Schroeder", and more recently, 

 Smith^^, have shown that both an irreversible change (decomposition ?) 

 and a reversible change gel sol takes jilace. At temperatures above 

 35° C. sol is stable, the gel form being stable below 15° C. When a 

 solution is cooled between these two temperatures the change sol 

 so gel does not take place at once but the equilibrium depends on 



