On the Proteid Reaction of Adamkiewicz, &c. 



23 



acid yielding a positive result is partially crystallised by freezing, the 

 power to yield the reaction is diminished in the crystals and increased 

 in the mother liquor. It is possible indeed, by repeated recrystallisa- 

 tion, to obtain glacial acid wholly incapable of giving the reaction. 



Much more readily, however, is the reactive substance to be con- 

 centrated by distillation. Any specimen of glacial acetic acid, if dis- 

 tilled, will yield the whole of any chromogenic substance it may contain 

 in the first runnings. After concentration to about half-bulk— more or 

 less according to the proportion of reactive substance originally present 

 — the residue will yield no trace of red or violet colour when mixed 

 with proteid and sulphuric acid ; while, on the other hand, the distil- 

 late twice or thrice fractionated yields the reaction with greatly in- 

 creased intensity.* 



It is easy to understand, therefore, why different specimens of acetic 

 acid obtained in the market yield the reaction with different degrees of 

 intensity, as this will depend upon the stage at which they were col- 

 lected during distillation in bulk. It is also clear why the reaction has 

 been looked upon by different observers as an uncertain one. 



The accepted view, that the colour phenomenon is due to the inter- 

 action of two chromogenic groups, both derived from the proteid 

 molecule under the action of the mixed sulphuric and acetic acids, is 

 certainly erroneous. One factor necessary to the reaction is supplied 

 by a substance admixed with the acetic acid. That it is in no sense a 

 furfurol reaction is indicated by the fact that the addition of furfurol 

 confers no power of yielding the colour with proteid upon a specimen 

 of acetic acid previously without it ; and, on the other hand, when 

 furfurol is added to acetic acid containing the chromogenic substance 

 in abundance, there is equally a complete absence of the reaction upon 

 mixing with strong sulphuric acid. 



II. Nature of the Substance responsible for the Reaction. 



Our earlier attempts actually to isolate the active substance from 

 acetic acid by fractional distillation were unsuccessful ; and, having 

 regard to the fact that, in a reagent so familiar as acetic acid, no admix- 

 ture could well have been hitherto overlooked unless the substances were 

 present in very small amount, we determined to seek first for indirect 

 evidence, such as might give at least some indication as to the kind of 

 substance we had to deal with. 



To this end we set out to add to acetic acid, previously deprived by. 

 distillation of its chromogenic admixture, various compounds of typical 

 constitution, in the hope that we might find among these some that 

 would yield at least an analogous reaction. 



* This applies to glacial acid ; with dilute acid of lower boiling point, concentra- 

 tion of the product by distillation is less easy. 



