1] UPON THE RATE OF GROWTH 325 



cal compounds used as food has been much more advanced^ 

 by studies upon this group than upon any other. Yeast 

 and bacteria have been especially investigated, but valuable 

 results have been obtained upon the higher fungi also. Con- 

 sidering all these results together, it appears that the nutritive 

 value of an organic compound is perhaps chiefly determined by 

 its assimilability; that is, the less the energy required to attack 

 and transform the compound by the various chemical means at 

 hand, the more favorable it is as a food. This assimilability 

 depends in turn upon the molecular structure upon a certain 

 molecular instability or lability upon the possession of that 

 quality which is found in its extreme expression in many 

 organic poisons. As LOE\V ('91, p. 761) has expressed it: 

 Poison action, like nutritive action, is a relative conception. 

 An indifferent body can become, by entrance of one atom-group, 

 a nutritive substance ; by entrance of an additional atom 

 group, a poison. While a certain lability that is, a certain 

 degree of ease of decomposition is a condition of the nutritive 

 quality of a substance, a slight increase of this lability can 

 give it a poisonous character, especially when the loosely 

 arranged atoms can link into that atom-grouping upon which 

 the vital movement in the protoplasm depends. Thus methan 

 is indifferent for bacteria, methylalcohol is a nutritive sub- 

 stance, formaldehyd is a poison, and its combination with sodic 

 sulphate again a nutritive material.* 



Additional laws of nutritive value which hold true in many 

 cases are as follows : The assimilable carbon compounds con- 

 tain the group CH 2 or at least CH. Under otherwise similar 

 conditions, compounds with one carbon atom are assimilated 

 with great difficulty (methylamin) or not at all (formic acid, 

 chloral) ; and, in general, but with important exceptions in the 

 case of certain classes of substances, the assimilability increases 

 as the number of carbon atoms in the molecule increases. The 



* The graphic formulas of these substances are : 



OH OH 



CH 4 CH 3 -OH CH 2 CH 2 



OH S0 3 Na. 



methan. methylalcohol. formaldehyd. formaldehyd- sodic sulphate. 



