298 PROTEINS 



violet colour. According to Cole,* the colour is due to the 

 presence of glyoxylic acid as an impurity in the ether. 



(vi) MoliscJis Reaction is a reaction for furfurol produced 

 by the action of concentrated sulphuric acid on a carbohy- 

 drate. The substance to be tested is treated with a few drops 

 of a 10 per cent alcoholic solution of a-naphthol ; concen- 

 trated sulphuric acid is then slowly added, when a red-violet 

 colour is formed at the junction of the two liquids. 



Microcheniical Reactions. 



The following are the usual microchemical tests employed 

 for the indication of proteins within the plant : — 



1. Iodine gives a yellow to brown coloration. 



2. With osmic acid a brown coloration results. 



3. Biuret reaction. — A solution of copper hydrate in caustic 

 potash may be added direct to the preparation ; or the section 

 may be steeped for some time, say twenty to sixty minutes, in 

 •2 per cent solution of potash, washed, placed in a 10 per cent 

 solution of copper sulphate for thirty to sixty minutes, washed 

 in water and mounted in a 2 per cent solution of caustic 

 potash. A mauve to violet coloration indicates the presence 

 of proteins, 



4. Millon's reagent. — The section or scraping is mounted 

 in a few drops of the reagent and warmed. A brick-red 

 coloration results when proteins are present. The reagent 

 may be prepared by dissolving some mercury in twice its 

 weight of nitric acid (sp. gr. i'42), the operation being per- 

 formed in a fume cupboard. When the action has ceased, the 

 solution is diluted with twice its volume of water. 



5. Xanthoproteic reaction. — A yellow to orange colora- 

 tion results with proteins. The preparation is warmed on the 

 slip with a few drops of strong nitric acid. The proteins 

 acquire a yellow colour which is changed to orange on 

 moistening with strong ammonia. 



PROTEINS AS COLLOIDS. 

 An important difference between the solution of an electro- 

 lyte and of a colloid lies in the fact that whereas the electrolyte 

 breaks up into two oppositely charged ions, the colloid appears 



*Cole: "Journ. Physiol.," 1904, 30, 311. 



