§ 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 Lobw ('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 CHg 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 excejDtions 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 



CH4 CH3-OH CHs CH2 



OH SOsNa. 



methan. methylalcoliol. formaldehyd. formaldehyd-sodic sulphate. 



