24 PLANT PHYSIOLOGY 



which represents only in simplest terms the nature of the 

 chemical changes 



C. H,, 6 = 2 C, H 6 + 2 CO, 



(alcohol) 



forming without oxygen two molecules of alcohol and two 

 of carbon-dioxide from one molecule of dextrose yields only 

 67 calories per gram-molecule.* 



The decomposition of one molecule of dextrose into one 

 molecule of butyric acid, two of carbon-dioxide, two of 

 hydrogen, which is accomplished by a considerable number 

 of bacteria and may be represented by the following reac- 

 tion 



C 6 H 12 6 = C 4 H e 2 + 2 C0 2 + 2 H 2 

 (butyric acid) 



yields about 75 calories per gram-molecule, f 



Bacteria forming acetic acid, acting on dilute solutions of 

 ethyl alcohol in the presence of free oxygen, partially oxidize 

 the alcohol and decompose it into acetic acid and water, 

 thus 



C, H.O.+ 0, = C, H 4 0, + H.O 



liberating 125 calories^ ; but if the alcohol were completely 

 oxidized, as in ordinary combustion, the reaction would be 



C 2 H 6 + 3 2 = 2 C0 2 + 3 H a O 



and the heat liberated would be nearly three times as much, 

 about 325 calories per gram-molecule. 



In these figures we have indices of the relative values of 

 complete and incomplete oxidations, and of oxidations and 

 decompositions, as sources of energy in the form of heat. 

 These figures are indices, to be trusted only so far as rela- 

 tive, not exactly proportional, values are concerned ; for the 

 chemist can control all the conditions under which he makes 

 a combustion in his laboratory and determines the number 

 of heat-units liberated ; he can so regulate the process that 

 there shall be no by-products and that no other compounds 

 are included in the reaction than those upon which he has 



* Rechenberg, 1. c., p. 66. t Ibid., p. 67. 



\ Quoted from Berthelot in Biedermann's Chemiker-Kalender for 1897, 

 p. 193 of the Beitrage. 



