WATER 77 



other substances combined, and it is no less 

 the chief excretion. It is the vehicle of the 

 principal foods and excretory products, for 

 most of these are dissolved as they enter or 

 leave the body. 1 Indeed, as clearer ideas of 

 the physico-chemical organization of proto- 

 plasm have developed it has become evident 

 that the organism itself is essentially an 

 aqueous solution in which are spread out col- 

 loidal substances of vast complexity. 2 As a 

 result of these conditions there is hardly a 

 physiological process in which water is not of 

 fundamental importance. 



All of these circumstances, which completely 

 justify the interest in water which Thales and 

 Aristotle, and nearly all later students of nature 

 have manifested, depend in great part upon 

 the quantity of water which is present out- 

 side the earth's crust, and upon its often 

 unique physical and chemical properties. 



1 Properly speaking, the entrance of the foods into the 

 body is across the wall of the intestine; at this point the 

 foods have all undergone digestion and are almost exclu- 

 sively in solution. In like manner excretion takes place 

 across the renal epithelium, or the epithelium of the lungs, 

 or across that of the sweat glands ; these too are traversed 

 only by substances in solution. 



2 "Der Organismus, Pflanze wie Tier, ist ein Gefass voll 

 wasseriger Losung, in dem sich als disperse Phase verschieden- 

 artige Kolloide befinden." — Bechhold, "Die Kolloide in 

 Biologie und Medizin." Dresden, 1912. 



