THE AMERICAN NATURALIST [Vol. LI 



because the energy changes do not favor the latter, or be- 

 cause in a given system the raw material for the auto- 

 catahi;ic reaction is absent, while that for the hetero- 

 catalytic reaction is present in abundance. 



However, the above considerations would lead us to be- 

 lieve that all substances should show some tendency to form 

 poh^neric molecules or crystals. This appears to conflict 

 with the classical division of substances into crystalloids 

 and colloids, but this division, like all others, can not be 

 expected to stand unmodified by the modern analysis. 

 There is plenty of evidence from direct observation that 

 many colloidal particles are simply very small crystals."^*^ 

 On the other hand, the molecules of polymeric substances 

 of high molecular weight, such as starch and certain pro- 

 teins, are probably of the same order of magnitude as 

 small colloidal particles. From the point of view of the 

 theory of matter, there is no fundamental difference be- 

 tween the general plan of a starch molecule and that of a 

 crystal of sugar, and it is highly probable that the dis- 

 tinction between colloids and crystalloids rests upon 

 purely quantitative relations, respecting the size of the 

 polymeric structures (cr\^stals) produced under ordinary 

 conditions. 



Large crystals are formed easily by simple substances 

 whose molecules have open fields of force or highly un- 

 saturated attractions. Small crystals are characteristic 

 of more complex sul)stances, common among the com- 

 pounds of carbon, having relatively closed fields. Large 

 mosaics of such molecules become unwieldy and are easily 

 disrupted by the temperature vibrations. They are also 

 built up more slowly than are mosaics of molecules with 

 open fields. The distinction between these two classes of 

 molecules is of course merely quantitative; no type of 

 molecule has a completely closed field, and on the other 

 hand no substance is capable of forming indefinitely large 

 crystals in a finite length of time. The atomic structure 

 of the solid phase of a colloidal gel is probably analogous 



30 See Ostwald, Wo., "A Handbook of Colloid-Chemistry," English trans- 

 lation, 1915, 56-66. 



