MICROCHEMISTRY 271 



Df alkaloids. On the other hand, if none of the above re- 

 agents produce precipitates, it is tolerably certain that there 

 are no alkaloids present. 



In examining plant tissues for alkaloids, Errera recom- 

 mends testing the fresh sections with alkaloidal reagents and 

 also sections which have been soaked in a five per cent alcoholic 

 solution of tartaric acid. In the second case no precipitate 

 should be obtained, owing to the extraction of the alkaloid. 



The final identification of the various alkaloids is usually 

 effected by means of colour reactions. 



Thus, if a section of the endosperm of Strychnos mux 

 vomica be mounted in a few drops of strong sulphuric acid, 

 the presence of strychnine is indicated by a red coloration 

 of the cell-contents. This colour will change to violet on 

 placing a small crystal of potassium chromate beneath the 

 cover-glass. 



Similarly, a section of the rhizome of Aconitum Napelius, 

 when treated with a few drops of 50 per cent sulphuric acid, 

 will show a carmine red coloration, due to the presence of 

 aconitine, in the parenchyma surrounding the vascular bundles. 

 This reaction is the more marked when the section has been 

 previously warmed in a solution of sucrose. 



These colour reactions are very numerous ; for them the 

 larger text-books and monographs must be consulted. 



Isolation. 



Most alkaloids do not occur free in the plant, but com- 

 bined with some acid in the form of a salt ; the acids most 

 commonly met with are tannic, malic, citric, succinic and 

 oxalic, while acetic and lactic acids are rarer ; some acids 

 occur only in connexion with certain alkaloids, such as 

 meconic acid with opium and quinic acid with quinine. 



In some few cases the alkaloids can be extracted from 

 their natural sources by means of organic solvents, such as 

 chloroform, carbon tetrachloride, ether, etc., but in the 

 majority of cases the alkaloid requires to be set free first by 

 the addition of an alkali, such as lime or baryta, since only 

 the free bases, and not the salts, are soluble in the above- 

 mentioned solvents. 



The material to be extracted is mixed with slaked lime 



