14 ENERGY TRANSFORMATIONS 



point of view of its function ; and we must study the chemical 

 and physical composition, the effect of the several constituents 

 upon one another, and upon the medium in which the cell lives, 

 the nature and action of the input and output of the cell, in- 

 cluding its secretions and how these are produced, the osmotic 

 phenomena and the effects of changes in the surrounding 

 medium, the characteristic accompaniments of stimulation of the 

 cell and of conduction of stimulation from part to part, and the 

 effects produced by cells possessing a life in common, as in the 

 multicellular animal. 



Physical chemistry affords us one of the most powerful ex- 

 perimental engines in conducting such inquiries for a reason which 

 has already been touched upon, namely, that the living cell is in 

 structure a complex solution containing both colloids and crystal- 

 loids, and that the chemical reactions occurring in the cell are 

 reactions in solution. Accordingly, although the whole matter 

 is profoundly affected by the fact that the cell is alive, it is 

 evident that our knowledge of cellular activities must be based on 

 knowledge of the properties of solutions, both of colloids and 

 crystalloids ; of reactions in solution, the velocity of such re- 

 actions and the conditions of equilibrium ; of the mutual effect of 

 crystalloids and colloids upon one another when in common 

 solution, and of the effects of the living cell as a peculiar energy- 

 transformer upon osmosis and diffusion. 



