256 GLUCOSIDES 



2. Cut a thick section of the fresh tissue to be examined 

 and place it in a 5 per cent alcohohc solution of potash for 

 about a minute ; transfer to a solution containing 2-5 per cent 

 ferrous sulphate and i per cent ferric chloride and keep at 

 about 60° C. for ten minutes. Place the preparation in a 

 dilute solution of hydrochloric acid— one part of strong acid 

 to six parts of water — for five to fifteen minutes. The presence 

 of hydrocyanic acid is. indicated by the formation of Prussian 

 blue. 



If leaves are to be tested, instead of cutting them up they 

 may be pricked all over with a bunch of fine needles and 

 then treated as above. 



3. Guignard's Test. — Dip strips of white filter-paper in a 

 I per cent solution of picric acid and dry ; before use moisten 

 the papers with a 10 per cent solution of sodium carbonate. 

 The test paper turns an orange red in the presence of fumes 

 of hydrocyanic acid. The test is very delicate, and the 

 rapidity of the change in colour depends on the amount of 

 prussic acid present, so that if the quantity be very small 

 the paper may have to be suspended in the test tube con- 

 taining the material to be tested, for some hours. 



This test has been modified by Waller so as to give quanti- 

 tative results, but it has been pointed out by Chapman * that 

 the coloration is due to reduction, and is, therefore, not speci- 

 fic for hydrocyanic acid ; accordingly the method must be 

 used with caution. 



Bishop t describes a convenient method for the estimation 

 of cyanogenetic glucosides. The material, leaves of the 

 cherry laurel for example, is treated with emulsin whereby 

 the prulaurasin is decomposed ; the hydrocyanic acid set free 

 is carried over by a current of air into potash which is then 

 titrated with silver nitrate. 



Some of the more important cyanogenetic glucosides may 

 now be considered. 



* Chapman : " Analyst," 1910, 35» 469- See also Francis and Connell : 

 " J. Amer. Chem. Soc," 1913. 35» 1^29. 



t Bishop : " Biochem. Journ.," 1927, 3i, 1162. 



