40 PROXIMATE CONSTITUENTS OF PLANTS 



with an unpleasant odour, recalling that of a foul tobacco pipe. 

 It is a powerful poison, and is used largely in horticulture as a 

 most efficient spray against certain insect pests, such as the 

 different forms of aphis affecting apples, damsons, hops, and 

 roses. 



A solution containing •075 per cent, of nicotine is employed for 

 spraying purposes. 



Fig. 10. — Apparatus for distillation with steam. 



23. Qualitative Reactions of the Alkaloids. 



The alkaloids, being of aminoid constitution, are characterised 

 by forming insoluble precipitates with certain reagents mostly of 

 an acid nature. These general precipitants may be conveniently 

 used for the separation of members of the alkaloids from solutions 

 containing them, the alkaloids being afterwards freed from the 

 precipitates by the action of caustic or carbonated alkalies. 



The general precipitants for the alkaloids comprise 

 the following : — 



Tannic acid, giving a white or yellowish precipitate. 



Picric acid, giving a yellow crystalline precipitate. 



Phosphomolybdic acid, giving a buff or yellowish 

 precipitate. 



