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TEE AMEBICAN NATURALIST [Vol. LI 



it might have been possible to raise a legitimate objection 

 to the illustration on the ground that the induced change 

 is not a chemical one. However, this objection is defi- 

 nitely disposed of by the recent work of the Braggs, and 

 others, on the constitution of crystals,^*^ which has shown 

 that the unit of structure in solid bodies is usually the 

 single atom, and not the molecule, since in crystals there 

 is, as a rule, no exclusive arrangement of the atoms into 

 molecular groups. The spacing of the atoms is such as 

 to make it clear, moreover, that the forces which hold the 

 total crystal system together are identical with those 

 which we regard as underhing chemical affinity. In other 

 words, in the crystal there is either no distinction between 

 inter-atomic and inter-molecular forces {i. e., between 

 chemical affinity and cohesion), or else the entire crystal 

 must be considered to be a huge poljTneric molecule.^i It 

 is therefore perfectly legitimate to treat the process of 

 crystallization as a chemical change, and to regard the 

 initiation of this process under the conditions above de- 

 scribed, as an example of autocatalysis, which may well 

 be typical. 



Although the results of crystal analysis indicate that 

 no distinct molecules are to be found in the solid state, 

 this is not true of the dissolved, or of the gaseous state. 

 Moreover, on account of the fact that their component 

 particles are held in place by forces of electrical attrac- 

 tion and repulsion, all molecules must possess their own 

 fields of electrical force, and the field of any molecule 

 must have a spatial form which is characteristic of that 

 molecule. These field patterns will thus be different in 

 the molecules of substances which ditfer chemically, and 

 will be similar in molecules of the same or of an allied 

 chemical substance.^^ The forces of cohesion in a crystal 

 may be thought of as resulting from the fusion of a large 

 numl>er of these molecular fields into a continuous mosaic, 

 and in sucli manner that tiieir several axes are parallel. 



22 See Comstock and Troland, loc. cit., &6-80. 



