CHAPTER VIIT. 



THE CHEMTSTKY OF LIFE. 



Organisms T HAVE stated in the last chapter, that the substance of 

 with -'- crystals is hard and impermeable by water ; that they 



crystals, grow by superficial accretion only, and are in a state of 

 perfect molecular immobility. The substance of organisms, 

 on the contrary, is mostly soft, and permeable by water; 

 they grow by interstitial accretion only, and a certain 

 degree of molecular mobility is the first condition of their 

 Accretion life. An organism, so long as it lives, is constantly losing 

 and waste, g^i^gtance by excretion, or waste ; and is constantly re- 

 ceiving new substance from without by accretion. Growth 

 is due to the excess of accretion over waste. 



But the constant parting with old material and acquiring 

 new material do not of themselves constitute life. If they 

 did, a glacier would be alive ; for it is constantly parting 

 with material by melting away below, while it is as con- 

 stantly acquiring new material in the shape of the snow 

 that slides down to it above. The word accretion describes 

 quite accurately the deposit of new material while a crystal 

 is growing ; but it is quite an inadequate description of the 

 acquisition of new material by an organism. Only sub- 

 stances of peciiliar chemical composition are capable of 

 becoming part of the living organism. Consequently, only 

 substances of suitable chemical constitution are capable of 

 being food for organisms. So far, the organism resembles 

 a crystal; for only the substance of which a crystal is 

 formed, or some other substance which is isomorphous or 

 plesiomorphous with it, can be added to the crystal and 

 become part of its structure. But the difference consists 



