PREFACE V 



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Why is there this constant change of atomic groups 

 between the living molecule and outside matter? It is 

 for the purpose of supplying the living molecule with 

 energy. It is probable that in the absorption of energy 

 by the living molecule, oxygen is released from its combina- 

 tion with carbon or hydrogen, and is attached to nitrogen, 

 while in the liberation of energy the reverse takes place. 

 Nitrogen seems to be the master element within the living 

 molecule. It is by virtue of its chemism that groups are 

 torn from non-living matter, taken into the living molecule, 

 and assimilated by atomic rearrangement; and furthermore, 

 it is on account of the lability of the compound thus formed 

 that potential energy is converted into kinetic and work 

 is accomplished. A nitrogen side-chain serves as a receptor 

 and transmitter of oxygen, and thus the traffic in energy 

 within the living molecule goes on rhythmically. It is not 

 to be supposed that the nitrogen side-chain, which serves 

 as the receptor and transmitter of oxygen, consists of so 

 simple a body as nitrogen or nitrogen oxide, but it is probably 

 a highly complex nitrogenous body in which the location 

 of the nitrogen is central, as suggested by Allen. Nor is it 

 probable that only oxygen is broken off from the pabulum, 

 but substances containing this element. This is the way 

 in which the living molecule keeps up its constant, rhythmic 

 traffic in energy, absorbing heat by assimilation, and giving 

 it off by dissociation. Each living molecule has not only 

 one, but many of these nitrogenous groups that act as 

 receptors. Moreover, metabolism within the molecule is 

 not confined to the absorption of oxygen, and the casting 

 out of non-nitrogenous products of combustion. The 

 whole molecule is labile, and there is probably in every 

 living molecule a nitrogenous, as well as a non-nitrogenous, 



