262 GLUCOSIDES 



shaken up alone, produce a froth, but if shaken in the presence 

 of oils, fats, or resins, they produce emulsions which are charac- 

 terized by their great stability. 



They are insoluble in absolute alcohol, ether, chloroform, 

 and benzene. 



From their aqueous solutions they may be precipitated 

 unchanged by the addition of ammonium sulphate. The 

 saponins are, as a rule, neutral substances, but a few have 

 feebly acid properties. Only a single saponin, namely, solanin, 

 has basic properties ; this substance, which occurs in Solanum 

 nigrum, S. dulcamara and in the fruit, eyes, and young shoots 

 of potatoes, owes its basic property to the presence of a nitro- 

 gen atom (see Nitrogen Bases), and appears to form a connect- 

 ing link between the saponins and the alkaloids. 



The neutral saponins are precipitated from solution by 

 basic lead acetate, while acid saponins are precipitated by lead 

 acetate. Similarly, barium hydroxide precipitates neutral 

 saponins in the form of their barium compounds (see below). 



The saponins are reducing agents, and will reduce am- 

 moniacal silver nitrate to metallic silver ; similarly, prolonged 

 boihng with mercuric chloride reduces this substance to 

 calomel ; saponins also blue a solution of potassium ferri- 

 cyanide containing ferric chloride, by reducing the ferric salt 

 to the ferrous condition, and so giving rise to the formation 

 of Turnbuil's blue. 



If boiled with acetic anhydride, alone or in presence of 

 sodium acetate or zinc chloride, the saponins are converted 

 into acetyl derivatives which are no longer toxic. On boiling 

 the acetyl derivatives with alcoholic potash the acetyl groups 

 are removed, but the resulting compound is not identical with 

 the original saponin. 



Isolation of Saponins. 



For their preparation, the raw material may be extracted 

 by means of hot alcohol ; in some cases the saponins separate 

 out on cooling the solution ; in other cases they may be pre- 

 cipitated from the alcoholic solution by the addition of ether. 

 For further purification the saponin dissolved in water is 



