MICROCHEMISTRY 269 



2. Mercuric iodide in potassium 



iodide, 



3. Tannic acid, 



4. Phosphotungstic acid, 



all of which 

 give colourless 

 amorphous precipi- 

 tates. 



Which give crystalline 



5. Auric chloride, | precipitates often 



6. Platinic chloride (see p. 276), j having characteristic 



. J melting points. 



The alkaloids are, however, not the only substances which 

 are thrown out of solution by these reagents, since most 

 nitrogen bases behave in a similar way, and the formation of 

 a precipitate is therefore not conclusive proof of the presence 

 of 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 mix 

 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 Napellus, 

 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 



