42 PROXIMATE CONSTITUENTS OF PLANTS 



Quinine. — (i.) Excess of dilute HgSO^ gives a marked 

 blue fluorescence. 



(ii.) Chlorine water or bromine water followed by 

 ammonia gives a green colour. 



(iii.) On moistening in an evaporating dish with 

 strong HCl, and warming over a Bunsen flame, a violet 

 colour and violet vapours are observed just before 

 charring takes place. 



Strychnine. — Mandelin's reagent (see above) gives 

 a bright violet coloration. 



Caffeine. — On evaporating to dryness with a little 

 bromine water, and moistening the yellow residue 

 with NH4OH, a bright purple coloration is produced 

 (Murexide test). 



Nicotine. — (i.) Formaldehyde and then nitric acid 

 gives a bright rose colour. 



(ii.) HgClg solution gives a white precipitate turning 

 yellow on standing. 



(/) The Glucosides. 



The glucosides are crystalline substances occurring in plants, 

 and which on hydrolysis, either by accompanying enzymes, or by 

 dilute acids, give various organic compounds together with glucose 

 (hence the name glucoside). Some glucosides give also hydro- 

 cyanic acid, and in this case the hydrolysis is called cyanogenesis 

 and the glucosides cyanogenetic. For example, the glucoside of 

 bitter almonds, amygdalin^ is hydrolysed by the accompanying 

 enzyme emulsin, to give glucose, hydrocyanic acid, and 

 benzaldehyde. 



A cyanogenetic glucoside is also found in linseed and linseed 

 cake (see section on Feeding-Stuffs, p. 155). 



As examples of non-cyanogenetic glucosides, salicin (occurring 

 in willow bark) and digitalin (occurring in seeds of the purple 

 foxglove) may be mentioned. 



24. Hydrolysis of Salicin. 



About 2 grams of salicin are boiled with dilute 

 sulphuric acid for a few minutes, whereby hydrolysis 



