CYANOGENETIC GLUCOSIDES 179 



and other plants, the glucoside has an acetone cyanhydrin 

 residue, while in the case of Prunus the residue is benzalde- 

 hyde cyanhydrin. The former glucosides are less stable than 

 the latter. 



With regard to the stages which lead up to the formation 

 of prussic acid and its compounds, Gautier has put forward 

 the supposition that it may possibly be formed by the action 

 of formaldehyde on nitrates, and this view is not inconsistent 

 with the distribution of nitrates in the leaves of some plants, 

 but nothing definite is known. 



Reactions, Microchemical ajid Otherwise. 



1. The presence of cyanogenetic glucosides or of free 

 hydrocyanic acid can generally be detected by chewing a small 

 piece of the material. 



2. Thoroughly crush the part it is desired to examine 

 under water and set it aside for some time, then filter and add 

 to the filtrate a little silver nitrate ; a white precipitate indi- 

 cates hydrocyanic acid, but this test must be used with 

 caution as many other substances give a white precipitate with 

 silver nitrate. 



If the amount of enzyme present in the tissue be very small, 

 the maceration must be allowed to proceed for some time, or 

 emulsin may be added to hasten the decomposition. 



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

 and place it in a 5 per cent alcoholic 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. 



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

 I per cent solution of picric acid and dry, moisten the papers 

 again with a 10 per cent solution of sodium carbonate and 



