ENERGY SUPPLY OF THE CELL 103 



milk or blood or on solid media. In these cases, the 

 single cell is the best unit. Since the determinations will 

 always be averages of many billions of cells, individual 

 differences will be eliminated entirely. The number 

 might be ascertained by plate count, direct count, 

 turbidity measurements or other methods. 



A simple method for studying the rate of fermentation 

 undisturbed by multipUcation of cells is to use such 

 large numbers of cells that they will not multiply. It is a 

 rule to use more cells than will naturally develop in such 

 medium. This method was suggested by Duclaux (1901, 

 vol. IV p. 328), and applied by Rubner (1904), Slator 

 (1906), and others. Rubner improved upon this method 

 by studying the alcoholic fermentation with large 

 quantities of yeast in pure sugar solution; since there 

 was no nitrogenous food available, the yeast cells could 

 not multiply to any appreciable degree. Slator (1906) 

 found that the presence or absence of nitrogenous 

 food did not influence the rate of fermentation. He 

 measured the decrease of vacuum through CO2 produced 

 by the yeast; this method made it possible to determine 

 the rate of fermentation in a very short time, five to 

 twenty minutes. Slator's technique was later slightly 

 changed by Rahn (1929) who measured pressure increase 

 instead of vacuum decrease. 



Another method which was used occasionally by 

 Chassevant and Richet (quoted after Duclaux, 1901) 

 was to add slight amounts of a weak disinfectant to the 

 culture, sufficient to suppress growth, but not sufficient 

 to retard fermentation. This method does not seem 

 very advantageous because the disinfectant is likely 

 to affect gradually the fermenting enzyme. 



An entirely different principle is involved in the 

 computation of the average number of acting cells by 



