124 PROGRESS OF SCIENCE IN THE CENTURY. 



In the first place, it has to be borne in mind that 

 by electrical discharges in air nitrogen is united 

 with oxygen to form nitric acid, and in a damp at- 

 mosphere the same agency causes nitrogen to combine 

 with water vapour to form nitrite of ammonia (Ber- 

 thelot). The rain after the thunderstorm brings 

 the products to earth. 



In the second place, it is stated by Schonbein that 

 wherever evaporation occurs minute traces of am- 

 monia are formed in the air. 



In the third place, the researches of Hellreigel 

 and Willfarth, repeated and confirmed by many, 

 show that leguminous plants can under the influence 

 of partner-micro-organisms, which form root-tuber- 

 cles, utilise (indirectly) the free nitrogen of the air. 



In the fourth place, the circulation of nitrogen and 

 the increase of availability is furthered by other lili- 

 putian agencies; namely, those soil-bacteria which 

 convert ammonia into nitrous acid, or carry the oxi- 

 dation further to the level of nitric acid. 



Foundation of Agricultural Chemistry. — If we 

 wish to associate any particular name with the recog- 

 nition of the fundamental fact of the circulation of 

 matter, it should be the name of Justus Liebig 

 (1803-1873). Himself a student under Gay-Lus- 

 sac, he became the master of one of the greatest 

 schools of chemistry, the initiator of chemical labo- 

 ratories, a pioneer of modern organic chemistry, one 

 of the prompters of chemical physiology, the founder 

 of agricultural chemistry, and the discoverer of many 

 important practical applications. 



The circulation of elements, of nitrogen for in- 

 stance, from the air or the soil into plants and 

 thence into animals, and thence bach to the soil or 

 air again, is a fact of great interest, justifying us 



