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6. CARBONIC ACID CANNOT SERVE AS FOOD. 



Various cxpcriincnts were made lo establisli wlial ma}- be the 

 volatile atmospheric carbon compouiid wliich renders the growth of 

 B. oUgocarho]}]ühis possible. That it cannot be carboiiic acid, whether 

 free or combined, resulted from the follow ing- experiments. In closed 

 culture-flasks with the best nutrient solutions, and arranged in sncli 

 a way, that at times a little free carbonic acid mixed with pui-e air, 

 could artificially be introduced, it was not possible to get any growth. 

 This experiment, which seemed of particular interest, has been so 

 frequently repeated, and so long continued under dilferent conditions, 

 that we consider it as quite certain, titat free carbonic acid cannot 

 serve for the nutrition of B. oligocarhopldhi.^. 



For testing the influence of combined carbonic acid, cultures were 

 made, firstly in the following solution : 



Tap- water J 00 



Dikaliumphosphate 0.01 



Kaliumnitrate 0.01 



Natriumbicarl)onate 0.1 



When cultivating at the free air surely a luxurious growth was 

 obtained, but it was by no means more vigorous than when the 

 bicarbonate was left out. 



If in this liquid the nitrate was replaced l)v an ammonium salt, the 

 result was quite the same. 



Secondly, the bicarbonate w^as replaced by common natrium car- 

 bonate, the same quantities of the different salts being used. But in 

 this case the action proved rather injurious than favorable. It is true 

 that the fdm had become considerable after a few months, but it 

 was directly to be seen that the gro\vth was so much inferior to that 

 of cultures obtained in the same circumstances but in absence of car- 

 bonate, that the determination of the permanganate number seemed 

 supei'fluous. Here, too, the replacing of nitrate l)y an ammonium salt 

 or by a nitrite caused no change. 



As a remarkable fact it may be mentioned, that in these experi- 

 ments, in our large flasks, containing a litre of air, the thin bacterial 

 film mounted very high up the dry glass-wall, which is likewise often 

 observed in the solutions made with distilled water, and may repose 

 on the absence of dissolved lime salts. 



If the tap-water was substituted by distilled water, the addition 

 of natrium carbonate did not cause an increase of bacterial growth 

 either. We found, for instance, in: 



