GENERAL 363 



and to distinguish between nitrite and nitrate bacteria 1 . These are all exceedingly 

 minute forms, and owing to their slow growth they are readily suppressed by other 

 bacteria when organic food materials are present. Each form has its own specific 

 oxidatory power which is always extremely pronounced, so that small amounts of 

 these organisms are able to produce large quantities of nitrites or nitrates as the 

 case may be. A large amount of the energy obtained is consumed in the assimila- 

 tion of carbon dioxide, for even under favourable conditions it requires the oxidation 

 of 33 to 37 parts of nitrogen, in the form of ammonia or nitrous acid, to yield the 

 energy necessary for the assimilation of one part of carbon 2 , and hence the 

 accumulation of organic material takes place but slowly. Under favourable con- 

 ditions Winogradsky (1. c., Nr. iv, p. 765) in sixty-five days obtained a crop 

 containing 22-4 milligrammes of carbon which would have probably weighed when 

 fresh 200 to 300 milligrammes. 



Winogradsky suggested that urea might be formed by the polymerization of 

 ammonium carbonate, whereas Hiippe and Loew 3 suppose that formic aldehyde 

 may be synthesized from carbon dioxide and water, but no empirical results have 

 as yet been obtained which indicate either of these substances as the primary 

 synthetic product. Nitrate bacteria assimilate carbon dioxide in the absence of 

 ammonia, and non-pigmented sulphur bacteria apparently derive sufficient energy 

 for the assimilation of this gas from the oxidation of sulphur or sulphuretted 

 hydrogen 4 . The possibility of obtaining energy by the oxidation of inorganic 

 compounds must be considered in all researches upon physiological combustion. 



PART III 



ABSORPTION OF ORGANIC FOOD 



SECTION 64. General. 



PLANTS which are unable to assimilate carbon dioxide must obtain all 

 their organic food- materials from without (heterotrophic or allotrophic 

 nutrition) ; by others a portion only of the organic food is drawn from 



1 Heraeus, Zeitschr. f. Hygiene, 1866, Bd. I, p. 210; Hiippe, Biol. Centralbl., 1887, Bd. VII, 

 p. 701 ; Chem. Centralbl., 1887, p. 1512 ; Winogradsky, 1. c. Further literature is given by Burri, 

 Centralbl. f. Bact., 1895, Abth. ii, Bd. I, p. 80. On the culture in organic nutrient solutions, and on 

 gelatinous silicic acid, cf. Winogradsky, I.e. ; on agar, 1. c., Nr. vii, p. 424; Beyerinck, Centralbl. f. 

 Bact., 1896, Bd. XIX, p. 258. Warrington (Journ. of Chem. Soc. 1878-9) and Schlosing and Muntz 

 (Compt. rend. Ixxxix. 1879) were the first to conclude that the process of nitrification was due to the 

 activity of micro-organisms. 



2 The heat coefficient of i gramme of NH 3 is 90,600 calories, for I gramme NHO 2 18,000 cal. 

 Stohmanri, Zeitschr. f. physik. Chem., 1890, Bd. VI, p. 355, and Ostwald, Lehrb. d. allgem. Chem., 

 1893, 2. Aufl., Bd. ii, p. 144. 



8 Loew, Centralbl. f. Bact., 1891, Bd. ix, p. 691. 



* Winogradsky, I.e., Nr. ii, p. 275; Bot. Zeitung, 1887, p. 547. Cf. Pfeffer, Energetik, 1892, 

 p. 208. It is uncertain whether other organisms, such as saprophytic fungi, &c., can obtain energy 

 by the oxidation of ferrous salts, &c. On iron-bacteria, cf. Winogradsky, Bot. Zeitung, 1888, p. 261 ; 

 Molisch, Die Pflanze in Beziehung ?. Eisen, 1892, p. 64. Cf. Sects. 23 and 96. 



