LECTURE XVI.] HISTORY OF CHEMISTRY. 349 



researches of Thiele, 80 who (amongst others) found out a con- 

 venient and technically practicable method for the manufacture 

 of hydrazine ; and to those of Raschig, 87 who cleared up the 

 nitrogen-sulphonic acids, and in doing so discovered the 

 method now employed for the production of hydroxylamine. 



It does not seem to me that this is the place to enter more 

 fully into this subject, since I am really giving a historical 

 sketch, in which only those things that are of general importance 

 can be prominently brought forward. 



I may thus recall here a discovery of HellriegePs which 

 marks an epoch in chemistry and agriculture. 88 According to 

 Hellriegel, leguminous plants, and lupins in particular, possess 

 the power of assimilating, with the aid of lower organisms, the 

 nitrogen of the air. In this connection the fact must not be 

 passed by without mention that Berthelot had previously 

 asserted the assimilation of free nitrogen. 89 



An observation which is to a certain extent of an opposite 

 character is the proof furnished by Buchner that fermentation 

 is possible even without living organisms, by means of the 

 liquid expressed from yeast (zymase). 90 



More particular consideration may be given to a research 

 by van 't Hoff, in which the idea and the significance of the 

 transition temperature are clearly stated. 91 Van 't Hoff is led 

 to the idea by the comparison of chemical reactions with the 

 transitions from one of the states of physical aggregation to the 

 others ; but the same conception may be arrived at by the aid 

 of the phase rule. 



Since the observations of St Claire Deville (see p. 302), the 

 phenomena of dissociation have been regarded and treated as 

 analogous to those of evaporation. Van 't Hoff now shows 

 that there are reactions which are comparable with the process 

 of fusion, and in which a fixed temperature marks the line of 



86 Annalen. 270, I ; 273, 133 ; Berichte. 26, 2598 and 2645 ; etc. 

 87 Annalen. 241, 161. 88 Hellriegel and Wilfahrt, Biederm. Centr. 18, 

 179. 89 Comptes Rendus. 106, 569. ** Berichte. 30, 117, mo, 2668, 

 etc. 91 Van 't Hoff and Deventer, Ibid. 19, 2142. 



