244 CHEMICAL INFLUENCES ON YEAST. 



reality, and the fermentative activity of the effective cells may 

 have been even stimulated, especially when a number of cells 

 in the crop became inoperative at a very early stage. Little is 

 known with regard to the effect on the capacity of the individual 

 cells. When working in a nutrient medium contained in a 

 hermetically sealed vessel of sufficient strength (e.g., a champagne 

 bottle), the yeast and its fermentative activity are soon brought 

 under the influence of carbon dioxide at high pressure, and will 

 suffer injury, not through the effect of the high gas pressure per 

 se, toward which yeast is not very sensitive, but owing to the 

 increased concentration of the acid in the fermenting liquid. It 

 is desirable that the pressure at which fermentation is finally 

 suppressed in closed vessels should be more accurately determined, 

 the limit not having been reached at 12.6 atmospheres in the 

 experiments made by C. G. MATTHEWS (III.) with Burton yeast 

 in beer wort. Conversely, the artificial removal of pressure from 

 the fermenting liquid, by drawing off the carbon dioxide (and 

 other volatile injurious products) accelerates fermentation and 

 heightens the attenuation (see chapter Ixiv.), a point already 

 observed by BOUSSINGAULT (I.), followed up subsequently by 

 PRIOR (II.) and others, and finally evolved by GRAUAUG and 

 KRANZ (I.), by NATHAN (in his Hansen apparatus), and by 

 Pfaudler into a vacuum fermentation process which has been 

 reported on by L. AUBRY (III.)- 



The most important points in connection with the influencing 

 of yeast by sulphur dioxide, whether in the gaseous state or in 

 the form of acid salts especially calcium bisulphite have already 

 been discussed on pp. 108, 109, vol. i. 



Though the comparatively slight action of arsenious acid and 

 its potassium and sodium salts on yeast investigated by C. 

 WEHMER (VII.) and C. KNOESEL (I.) is of no importance in 

 practical fermentation, it has proved useful in the theoretical 

 study of the enzyme of alcoholic fermentation (see chapter Ixiii.). 



Hydrochloric acid, sulphuric acid, hydrofluoric acid, and certain 

 other fluorine compounds have a relatively stronger effect on most 

 bacteria than on yeasts, so that, by selecting an appropriate 

 degree of concentration, the latter organisms can be protected 

 against the former (see vol. i. p. 248). C. KNOESEL (I.) has 

 shown that an acid nutrient medium is not unconditionally 

 essential to yeasts ; but, within certain limits, the presence of 

 free acids stimulates both reproduction and fermentative 

 activity. 



That boric acid has little adverse influence on yeast was observed 

 by J. MATTERN (I.) and E. BIERNACKI (I.); and in H.WILL'S 

 experiments (I.) it proved unable to kill the whole of the cells, 

 even after an exposure of 20 minutes. The same has been found 

 in respect of calcium borate, and also contrary to a previous 

 report by WERNKE (I.) of borax, so that use can be made of 



