44 



moist earth with respect to chlorine gas. (3) Swanson^' and Hulett : — 

 Estimation of gases in eflflueuts by partition between vacuum and 

 liquid — assumption that Henry's Law holds true for such solutions. 



(d) Application to Brewing. 



In the brewing industry it has long been recognised that a Icnow- 

 ledge of the carbon-dioxide equUibrium is of' the first importance in 

 connection with the sprakling quality, persistency of head and 

 palatabUity of beer. After Langer^^ and Schultze's experiments it 

 was for a long time thought that the solubility of carbon-dioxide in 

 beer was greater than the solubihty in a corresponding water-alcohol 

 solution. Prior**', modifying the method of investigation, considered 

 that increased absorption was due to presence of phosphates. The 

 mean phosphate content of beer, however, is about • 07 per cent. ; 

 insufficient to explain any considerable increase in carbon-dioxide 

 solubility. The assumption that an ester of carbon-dioxide and 

 alcohol is formed, has been shown by Mohr^'^ to be without foundation 

 as also was the suggested combination with proteids. Mohr concluded 

 that the gas was merely held in suspension to an extent determined 

 by the viscosity. On the basis of Langer's experimental data, 

 Emslander^' and Freundlich, considered that the increased solubihty 

 was due to adsorption of the gas by the colloids — chiefly dextrin and 

 albuminoids. These colloids have each been found to lower the 

 solubihty of carbon-dioxide in water; and alcohol also lowers the 

 solubihty (Miiller*"). It is not surprising therefore that Findlay and 

 Shen^8 found for wort and different grades of beer that, contrary to 

 the usually accepted view, the carbon-dioxide solubility was con- 

 siderably less than for water, and independent of pressure. Increase 

 in alcohol content, with corresponding increase in total colloids,* 

 was shown, also, to decrease the solubility, and that even allowing for 

 the alcohol, the solubihty was much less than in water. It is evident 

 that the method used by the earher investigators gave the total 

 amount of carbon-dioxide in a beer which was supersaturated to an 

 unloiown degree. Distinction must therefore be drawn between true 

 solubihty, and degree of supersaturation, and to understand the 

 problem better the second aspect of gas equilibrium must be 

 considered, namely : — 



II. — Rate of Evolution of Oas. 



It is well known, although not always sufficiently well recognised, 

 that gas supersaturation readily occurs in aqueous salt solutions 

 (Lamplough^^), and in ordinary brewing practice during fermentation 

 in cask, the beer becomes supersaturated. It was shown by Findlay 

 and King^2 that certain colloidal solutions supersaturated'with carbon- 

 dioxide remain quiescent, sometimes over a period of 20 minutes. 

 In this condition of metastabUity the solution is exceedingly sensitive, 

 a very slight mechancial shock being sufficient to cause gas evolution 

 (c.f. Young^^). Such quiescence only occurs if scrupulous care is 

 taken in the cleaning of the api^aratus and filtration of the solutions 



* According to Marc colloids are proportional to total solids present in 

 beers. 



