THE DAWN OF THE BIOLOGICAL THEORY 209 



It may be seen that in all the early theories it was supposed 

 to be a purely chemical process; it was not until the time of Pasteur 

 that the biological explanation for the formation of nitrates received 

 any support. 



It is interesting, however, to note the careful work of Bous- 

 singault in the years between 1860 and 1878, on the natural occur- 

 ring saltpeter beds, especially those of Peru and Ecuador. He 

 raised the question: "Have not the nitrates in these natural deposits 

 resulted from the breaking down of organic substances rich in 

 nitrogen?" for it had long been the practice to use blood, urine, 

 and other animal refuse for the production of nitrates. For this 

 reason, Boussingault did not think it likely that the gaseous nitro- 

 gen of the air played a very great part in the process of nitrifica- 

 tion. In order to test this, he placed soil with known nitrogen 

 content in 100-liter jars and allowed them to remain for eleven 

 years. At the end of this time they were analyzed, and, in spite 

 of the fact that a very active nitrification had taken place, there 

 was no increase in the total nitrogen of the soil. From this he 

 concluded that free nitrogen takes no part in the formation of 

 nitrates, but that they result from the organic matter of the soil. 

 In another set of experiments he added organic manure to soil, 

 sand, and chalk, and left them to nitrify. He obtained an active 

 nitrification in the soil, but none in the sand or chalk. This fact 

 had a great influence on the old theories of nitrification. Why this 

 difference if the soil acts merely as a catalyzer? It may be said 

 that this w:as the beginning of the end of the old chemical theories 

 of nitrification. 



The Dawn of the Biological Theory. At this time (1878) the 

 work of Pasteur was beginning to take firm root. There had 

 appeared a series of reports on fermentation, one of the earliest 

 (1862) being on the fermentation of acetic acid. This was similar 

 to nitrification, for it was known that alcohol could be oxidized 

 to acetic acid by use of the platinum sponge. In fact in this early 

 publication Pasteur suggested that nitrification was due to a fer- 

 ment, and soon after Mliller observed that the ammonia in sewerage 

 is rapidly changed into nitrates, but no corresponding change takes 

 place in pure ammonia solution. He suggested that the sewage 

 probably contained a ferment which was absent from the pure 

 solution prepared in the laboratory. He, however, took no steps 

 to prove that the process was a true fermentation. Between 

 1871-75 Gilbert noted that the drainage waters from the Rothamsted 

 experiment fields contained more nitrates as the ammonium salts 

 applied to the soil increased. 



To the chemists, Schlosing and Miintz, belongs the credit of 

 establishing by experiment the fact that nitrification is a biological 

 process. They were trying to ascertain if the presence of humus is 

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