PHYSIOLOGY OF THE MICROBES 75 



modifications in the number and quantity of the constituents, 

 to compare the weights of the growth obtained, to determine 

 the temperature and the hygrometric condition of the atmos- 

 phere, and even to take account of the shape of the culture 

 flasks, which affected the oxygen supply. 



If potassium is cut out from the above formula, the crop of 

 aspergillus is 25 times less, all the other conditions remaining 

 the same; if the figure 25 is taken as representi^ig the 

 measure of utility of potassium, the utility of the other food- 

 stuff may be represented as follows : nitrogen, 153 ; phosphorus, 

 182; sulphur, 25; silicon, i"4; magnesium, 91; zinc, 10; 

 iron, 27. 



If you take the ratio between the weight of one of these 

 food-constituents and the weight of the crop due to its 

 presence, you get a number which expresses its specific utility, 



22'5 



e.g., for zmc = i;6o. 



■^ o'o4 -^ 



This ratio was in several experiments found to be 953. 

 For nitrogen it is 17; for phosphorus 157; sulphur, 346; 

 potassium, 64; magnesium, 200; iron, 857. It is to be noted 

 that aspergillus takes up zinc from a medium in which the zinc 

 is diluted i in 50,000. 



If I in 1,600,000 of silver nitrate is added to the fluid, 

 .aspergillus spores no longer sprout in it : even when simply 

 poured into a silver vessel the fluid dissolves sufficient silver 

 to prevent growth. The plant is thus an indicator so sensitive 

 as to mark, by its refusal to sprout, such small quantities as 

 i/24oth of sulphate of copper, i/8,oooth of platinum bichloride, 

 i/5o,oooth of perchloride of mercury. 



Zinc has a definite food value but iron acts differently, 

 namely, by neutralizing a substance which is produced by the 

 growth and becomes injurious to it, perhalps sulphocyanic acid. 



Tartaric acid maintains the acidity of the medium and 

 prevents bacteria from contaminating the cultlire, for almost 

 all bacteria refuse to grow except in an alkaline medium. 

 Hence a culture of aspergillus in Raulin's fluid produces its 

 own asepsis. The sugar is only assimilated after inversion by 



