( ^05 ) 



medium, and not in llie depth, the Ihickneös of the Layer of tlie 

 nutrient solution and consequently its volume, is, as already observed, 

 actually indifferent. That is to say, by enlarging the surface of the 

 solution, a bacterial film of any dimensions is to be obtained, which 

 circumstance is of importance for appreciating the prcxbiclix iiy of a 

 certain quantity of a nutrient solution, the more so as I he thickness 

 of the bacterial film is usually only one cell-layer. How vcrv thin 

 the required thickness of this layer can be, gi-owlh being still 

 possible, may be derived from the fact, that, especially ^\ hen using 

 distilled water with nutrient salts, the tilm can nionnl at (he appa- 

 rently dry glass-wall from 1 to 1.5 decimeter iiigh, and nol seldom 

 extends on it nearly to the cotton plug. Only in certain A-inegar 

 bacteria I observed the same. 



As it seems that our bacterium forms no compounds prejudicial 

 to its growth, so the only circumstance, which governs its increase 

 relatively to a given volume of liquid, provided its surface be of a 

 sufficient extent, is the lack of one or more elements necessary for 

 the nutrition. Carbon cannot be among the number, our expei-iment'^ 

 l)eing made \\itli free entrance of air. 



Although it is thus established, that only the number of bacteria, 

 produced in a certain time i>er surface-unit, indicates the rate at 

 which the atmospheric carbon is assimilated, we will yet gi^e the 

 quantities in relation to the volume of the solution, because then a 

 comparison can be better made with the numbers found by other 

 authors for polluted waters. 



5. HOW MUCH CARBON IS ASSIMILATED. 



First we determined by an experiment, in which, after ^■igorous 

 shaking, a culture \\as divided into t\vo equal portions, how much 

 one half contained at direct weighing, of bacterial substance, whereas 

 the other half was titrated with kaliumpermanganate. We used for 

 this a three months old culture on : 



Tap-water iOO 



Na.HPO, 0.02 



KCl 0.02 



KNO3 0.02 



The fdm from the part, destined for the weighing, was separated from 

 the liquid by filtration, washed out on the fdter with strong! v 

 diluted hydrochloric acid, and subsequently with distilled ^vater, to 

 remove the chlorids. Subsequently the filter with the dim was 



27 



Proceedings Royal Acad. Amsterdam. Vol. V. 



