122 REPORTS ON THE STATE OF SCIENCE.—1918. 
The theory of dyeing has been described in an earlier report by 
King (1917) to which the reader may be referred. There are many 
factors which play a part in addition to chemical composition. 
Fischer (1899) bas shown that the same substance stains differently 
according to the size of the particles in which it isfound. The sign 
of the electric charge is aiso of importance. The meaning of the 
facts is still obscure. Nervous substance has a special “ affinity ” for 
methylene-blue and other thiazine dyes, but also for some safranin- 
azo dyes, which have no chemical relationship to the former. 
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