Imiller] chemical LABORATORY RESEARCHES 269 



to be the case. The constant temperatures may be explained by a 

 consideration of the magnitudes of the heats of melting and re- 

 crystallization and the velocity with which the melting and the re- 

 crystallization take place. 



The Effect of Acids on the Rate of Reproduction of Yeast 



By Miss E. Taylor, B.A. 



Presented by Professor W. Lash Miller, F.R.S.C. 



Nitric, hydrochloric, sulphuric, acetic, oxalic, tartaric, glycollic, 

 lactic, or chloracetic acids were dissolved in various proportions in 

 wort, or in mixtures of wort and sugar-salt solutions, or in various 

 artificial media containing different preparations of bios. In each 

 case the PH value was determined, and the rate of reproduction of 

 yeast under suitable conditions of temperature and stirring. The 

 PH values at which reproduction is checked are very different for 

 the different acids in the same nutrient solution, and also for the 

 same acid in different media. 



The Quantitative Determination of Bios 



By G. H. W. Lucas, B.A. 



Presented by Professor W. Lash Miller, F.R.S.C. 



When a small seeding of yeast is introduced into a nutrient 

 solution consisting of ten per cent, of wort and ninety per cent, of 

 a sugar-salt solution, and the whole is shaken in a thermostat at 

 25°C., the number of yeast cells increases logarithmically until it 

 reaches about 145 million cells per cubic centimeter and then remains 

 constant. As the alcohol concentration when reproduction ceases is 

 too low to affect the yeast, the cessation of reproduction was ascribed 

 by Mr. Clark^ to exhaustion of the bios from the nutrient solution. 



^Trans. Roy. Soc. Can., 1921, Sec. HL, pp. 47; and Jour. Phys. Chem., vol. 

 26, pp. 42-60. 



