40 CELL-CONTENTS AND CELL-WALLS. 



Several other reactions are due to the presence of tryptophane, 

 e.g. (1) proteins give a reddish violet colour, afterwards turning 

 brown, when heated with strong hydrochloric acid ; (2) proteins give 

 a blue colour when precipitated by alcohol, then washed with ether, 

 and treated with strong hydrochloric acid ; (3) proteins give a green 

 or blue colour when heated with benzaldehyde, a drop of ferric 

 chloride, and strong hydrochloric acid. 



(g) Molisch. Reaction. Add a few drops of a-naphthol solution 

 to the albumin solution, shake up, then run some strong sulphuric 

 acid to the bottom of the tube. A violet ring is formed at the junc- 

 tion of the two liquids. This reaction is of special interest, since 

 it depends upon the carbohydrate radicle present in protein (see 

 63, g). In the reaction, furfural is formed from the carbohydrate 

 radicle. 



In addition to the preceding colour reactions of proteins (a to g) , 

 some of the chief precipitation reactions may now be studied 

 (h to r). 



(h) Coagulation by Heat. Heat some albumin solution. Since 

 its reaction is alkaline, no clot is formed, but only an opalescence or 

 perhaps a slight precipitate on the inside of the tube, (i) Now 

 slightly acidify another portion of the albumin solution by adding a 

 few drops of dilute acetic acid, and heat. The solution turns cloudy, 

 and then a precipitate of coagulated albumin is formed ; note that 

 this precipitate is not soluble in cold acids and alkalis, but gradually 

 dissolves on heating with caustic soda, (j) Make some of the 

 solution faintly acid ; immerse the test-tube in a beaker of water, 

 with a thermometer, and heat gradually the tube may be fixed in 

 the clamp of a retort stand, so that it dips into a beaker of cold 

 water placed on a sand-covered plate over a Bunsen or spirit lamp. 

 Note the temperature at which a cloudiness appears in the solution, 

 and when (usually about 70 C. ) coagulation becomes complete. 



Note that a precipitate of coagulated albumin is given on adding 

 each of the following reagents to portions of the solution : (k) 

 alcohol ; (/) nitric or hydrochloric acid ; (m) mercuric chloride 

 solution ; (n) lead acetate solution ; (o) tannic acid, or strong tea 

 that has been stewed for about half an hour. 



(p) A white precipitate is formed not given with peptones on 

 adding a little glacial acetic acid, then potassium ferrocyanide 

 solution drop by drop. 



(q) Add excess of acetic acid, then an equal volume of saturated 

 sodium sulphate solution, and heat ; the precipitate formed removes 

 all proteins (except peptones) from a solution. 



(r) Saturate the solution with ammonium sulphate, by adding 

 crystals or the powdered salt until no more will dissolve on shaking 

 a white precipitate, not given with peptones. This throws down 

 all proteins (except peptones) from solution ; filter, and note that 

 the filtrate now contains no proteins 



