Relations and Transformations of Energy 45 



a saturated or electrolyzed solution, the movements of a Mimosa 

 leaf, or the contraction of an animal muscle. 



Verworn has equally realized the necessity for such an exten- 

 sion of the term in the realm of animal physiology. For in 

 his lectures on "Irritability" (p. 1) he says: "Irritability is a 

 general property of living substance but not exclusively so. 

 Irritable systems also exist in inanimate nature. What char- 

 acterizes living substances is not irritability as such, but an 

 irritability of a specific type." 



The consideration of Nutrition and Growth presents a more 

 complicated problem. But what would have seemed abso- 

 lutely discontinuous processes before Graham's discoveries are 

 steadily becoming steps in a consecutive chain of events. For, 

 while crystalloids show no phenomenon comparable to organic 

 nutrition in adding to their substance, the group of colloids 

 presents some remarkable and exact similarities. Thus inor- 

 ganic and organic colloids alike consist of complex molecules 

 of varying size, but each surrounded by a jacket of water. 

 Both show a high degree of mobility and elasticity, according 

 to the amount of water present at any moment. Both show 

 capacity for formation of membrane pellicles round enclosed 

 contents — either crystalloid or colloid — of more or less active 

 osmotic capacity. Both show elastic expansion of, and capac- 

 ity for, forming additions to such membranes, through con- 

 tinued absorption from surrounding liquids of substances that 

 add to the amount of the enclosed materials. Other homologies 

 might be mentioned, but for present purposes such would be 

 superfluous. We therefore consider with Leduc (p. l29) that 

 exact cases of nutritional and growth continuity have been 

 established in both. So to speak of the nutrition and growth 

 of some inorganic colloid mixtures or unions is as appropriate 

 as if we spoke of those in a yeast cell or an amoeba. 



Respiration is an unceasing and important phenomenon of 

 organic types that seems, with our present knowledge, scarcely 

 if at all referable to inorganic bodies. If the student of colloid 

 chemistry could demonstrate that in growths like Traube's 

 cells, or in mixtures of several nearly equilibrated bodies, a 



