NITROSO-BACTERIA AND NITRO-BACTERIA 289 



consideration by several workers, inter alia by ADAMETZ (IX.) and A. B. Frank ; 

 nevertheless, the result did not fulfil expectations. The last-named German 

 mycologist then contradicted the assumptions of the two French agricultural 

 chemists, and championed the views held by Dumas. To this revival of an old 

 hypothesis we owe the production of a comprehensive work by H. PLATII (I.), 

 which is commended to the attention of the reader not only on account of the 

 new discoveries it mentions, bub also because the first part contains a collection 

 rich in information for the chemist of all the then known methods for the 

 production of nitric acid from ammonia by oxidation. In the second part of 

 this treatise it was stated, on the basis of new experiments, that completely 

 sterilised soil no longer possesses the faculty of converting ammonia into nitric 

 acid. It was furthermore shown that, when organisms are entirely excluded, 

 neither the soil as a whole, nor any one of its constituents, is capable of trans- 

 forming ammonia into nitric or nitrous acid by occluding atmospheric oxygen. 

 A re-examination of this work by H. LANDOLT (I.), who undertook the task in 

 consequence of an objection raised by A. B. FRANK (IX.), led to a complete con- 

 firmation of Plath's discovery on all points. It was thus ascertained (in 1888) 

 by the exclusion method that in the oxidation process now under our notice the 

 role of oxygen-carrier is played by living organisms, arid that consequently 

 nitrification is a physiological process. 



204. Nitroso-Baeteria and Nitro-Baeteria. 



The discovery and closer investigation of these unknown organisms was 

 shortly afterwards effected by S. WINOGRADSKY (VII.). It is not surprising 

 that their preparation as pure cultures was so long delayed, when we remember 

 that these bacteria do not thrive on media rich in organic nutrient substances. 

 The above-named Russian physiologist successfully employed for this purpose 

 the gelatinous inorganic substance, viz., precipitated silica, recommended by 

 W. KtiHNE (I.). When prepared by precipitation from water-glass (alkali 

 silicate) by hydrochloric acid, and purified by dialysis, concentrated by boiling, 

 and then sterilised in the steamer, this silica forms a vitreous mucinous mass. 

 This is then incorporated with a sterilised solution of the sulphates of potash, 

 magnesia, ammonia, and carbonate of soda, inoculated with the bacterial sample. 

 These salts cause the silica to set, so that the germs in the sowing are fixed 

 separately, and thus may be kept apart, even when they have developed into 

 colonies. In this manner Winogradsky primarily succeeded in obtaining 

 cultures of assured purity, by means of which he was enabled to arrive at 

 conclusions unattainable by the fractional sowings and dilution method em- 

 ployed by previous workers, e.g. W. HERAEUS (I.), P. FRANKLAND (IH.) and 

 E. WARINGTON (III.). 



One of the weightiest of these results is the fact determined by WINO- 

 GRADSKY (VIII.) that the numerous species of the group of nitrifying bacteria 

 are classifiable into two sharply divided sub-groups : nitroso-bacteria and nitro- 

 bacteria. 



The nitroso-bacteria oxidise ammonia to nitrous acid, in accordance with the 

 equation 



(NH 4 ) 2 + 3 3 = N 3 3 + 4 H,0, 



but no farther. For this reason nitrites are not altered by these bacteria, 



On the other hand, the nitro-bacteria lack the faculty of attacking ammonia, 

 but perform the task of converting nitrous acid into nitric acid, in accordance 

 with the equation 



I T 



