376 THE NITRIFYING BACTERIA. 



higher the reaction becomes weaker, and ceases altogether at 55 C. 

 It proceeds the more rapidly as the degree of moisture in the soil 

 increases, provided aeration is not thereby impeded. A faintly 

 alkaline reaction facilitates the progress of nitrification, which, 

 moreover, may not always result in the production of nitrates, 

 but at times does not extend beyond the formation of nitrites, 

 especially at low temperatures (below 20 C.) and with a restricted 

 admission of air. 



Both workers also endeavoured to obtain pure cultures of the 

 organisms under investigation. The result of their endeavours 

 will not be judged too harshly when the existing lack of any 

 reliable method of pure cultivation at that time is remembered. 

 When the introduction of the Koch gelatin plate afforded a new 

 appliance for this purpose, it was pressed into the service now 

 under consideration by several workers, inter alia by ADAMETZ (IX.) 

 and A. B. Frank ; nevertheless, the result did not fulfil expecta- 

 tions. The last-named German mycologist then contradicted the 

 assumptions of the two French agricultural chemists, and cham- 

 pioned the views held by Dumas. To this revival of an old 

 hypothesis we owe the production of a comprehensive work by 

 H. PLATH (I.), which is commended to the attention of the reader 

 not only on account of the new discoveries it mentions, but 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 con- 

 verting 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 transforming 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 confirmation 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, and that consequently nitri- 

 fication is a physiological process. 



204. Nitroso-Baeteria and Nitro-Baeteria. 



The discovery and closer investigation of these unknown organ- 

 isms 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, recom- 



