128 LECTURE VIII. 



plant as manure. Further Boussingault himself observed 

 incidentally that a Helianthus which had been manured with 

 ammonium carbonate had gained three times as much nitro- 

 gen as one which had been supplied with nitrates during an 

 experiment. 



Naegeli finds that Fungi can assimilate nitrogen in organic combina- 

 tion in the form of proteids, of methylamine and other amines, in addi- 

 tion to the substances mentioned on p. 124. He concludes that they 

 assimilate nitrogen most readily when it is supplied to them in the form 

 of NH 2 ; less readily as NH ; with difficulty as NO ; and not at all as CN. 



It appears that nitric acid may be absorbed in the form of 

 any salt which can diffuse into the plant, the most common 

 bases being soda, potash, lime, magnesia and ammonia ; 

 ammonia may be absorbed in combination as chloride, nitrate, 

 sulphate, or phosphate, but the carbonate is injurious, at least 

 in water-cultures, on account of its alkalinity. 



Inasmuch as all plants necessarily absorb their nitrogen in the com- 

 bined form, it will be of interest to enquire into the sources of combined 

 nitrogen. It has been known since the time of Cavendish that a forma- 

 tion of nitric acid attends electric discharges in the atmosphere, and it 

 was thought that this might maintain the supply of combined nitrogen in 

 the soil. But Lawes and Gilbert have found that the quantity thus formed 

 and conveyed to the soil by rain will only account for a fraction of the 

 quantity removed from the soil in the crop (in the absence of nitrogenous 

 manure), so that we are compelled to seek for other sources of supply. 

 These have been suggested by various observers. Thus Schonbein 

 pointed out that a formation of ammonium nitrite takes place when water 

 evaporates at certain temperatures, and that this substance is also formed 

 when various substances, such as phosphorus, fats, etc., are burned in 

 moist air, but this has been shewn to depend upon the oxidation of ammo- 

 nia. Still it must not be overlooked that a formation of nitrogen-com- 

 pounds has been found to attend various processes of oxidation ; Kolbe 

 and Hofmann observed that when a hydrogen flame was kept burning for 

 a time in a vessel, the water produced always contained nitric acid, and 

 Bunsen observed that when a mixture of air and hydrogen was exploded 

 by the electric spark, the resulting water contained nitric acid. This is 

 also the case when phosphorus is burned under a bell-jar. An important 

 source is probably to be found in the direct combination of nitrogen with 

 organic substances, the combination being dependent upon a slow electri- 

 cal discharge. Berthelot found, for instance, that when cellulose and 

 dextrine are exposed in air for some hours to the action of a weak electri- 

 cal current, a substance is obtained which evolves ammonia when heated 



