210 



Presents, 



[Feb. 20, 



sine, and hydrocyanic acid. It thns appears probable that the colour 

 reaction of the proteids that occurs on addition of a cupric salt and 

 an alkali is due to the existence in the proteid also of cyanogen. 



Jnst as some proteids give a rose-red colour, and others a violet, so, 

 in the list of substances just enumerated, some give a rose-red, and 

 some a violet. Biuret was the substance in which a rose-red colour 

 was first noted ; hence the term biuret reaction, as applied to peptones. 

 Cyanuric acid was the substance in which a violet colour was first 

 noted. Probably in both cases the reaction is dne to a cyanogen 

 radicle ; but the cause of the difference in colour is unknown. In the 

 same way, our ignorance of the constitution of the proteid molecule 

 stands in the way of our discovering the difference between peptones 

 that give a rose-red colour and albumins that give a violet colour. 



The term biuret reaction is to some extent a misnomer, as applied 

 to the peptones and albumoses ; the test with the modification I have 

 introduced behaves a little differently in the two cases. The substance 

 that peptone most nearly resembles in its colour reactions is hydro- 

 cyanic acid, as is shown in the table (p. 209), in which a contrast is 

 drawn between the chief substances which I have examined. 



Using the word cyanogen in the widest possible sense, the con- 

 clusion I should draw from such a series of experiments is that the 

 colour reaction with a cupric salt and an alkali is a cyanogen 

 reaction. Among the simpler organic bodies examined, we have 

 certain cyanogen compounds, like cyanuric acid, that give a violet 

 colour ; and certain proteids (the albumins and globulins) give the 

 same colour. There are certain other substances, like biuret, which 

 give a red colour without any intermediate violet stage; there are 

 others, like hydrocyanic acid, which give a violet colour with 

 ammonia, which is turned red by potash or soda; and to this last 

 group the peptones also belong. Just as there is a different combina- 

 tion of the cyanogen in cyanuric acid from that in hydrocyanic acid, 

 so there is probably the same difference between the combination of 

 the cyanogen in albumin and peptone respectively ; and this difference 

 is, as a rule, brought about by a digestive ferment. 



Presents, February 20, 1890. 



Transactions. 



Bergen : — Museum. Aarsberetning. 1888. 8vo. Bergen 1889. 



The Museum. 



Birmingham : — Mason Science College. Calendar. 1889-90. 8vo. 



Birmingham. The College. 



Philosophical Society. Proceedings. Vol. VI. Part 2. 8vo. 



Birmingham [1889]. The Society. 



