294 VETEEINAEY TOXICOLOGY 



alkalised, and the organic base again extracted, now in a 

 purer form. 



Acids are purified by solution in an alkali (soda or 

 ammonia), extraction of the alkaline solution with solvents, 

 acidification and re-extraction of the acidic component. 



Solution in water, alkali, or acid and re-extraction, often 

 efi'ect a partial purification of neutral substances. 



Alkaloids may often be precipitated from solution by 

 means of an alkaloid reagent, such as phosphomolybdic 

 acid, and the precipitate filtered and washed. It is then 

 decomposed — e.g., with soda or baryta, and the alkaloid 

 extracted with solvents. 



Basic lead acetate precipitates proteins and many other 

 impurities from organic solutions. After filtration, the 

 excess of lead is removed by a current of sulphuretted 

 hydrogen, and the organic substances are separated by 

 solvents, or by evaporation, from the filtrate. ■ 



General Reactions of the Org-anlc Poisons. — There are 

 certain reactions by means of which it may be decided 

 whether a substance is or is not an alkaloid or a glucoside. 



Alkaloids. — There are numerous substances which give 

 precipitates with alkaloids — e.g., phosphomolybdic acid, 

 bismuth potassium iodide, iodine in potassium iodide, tannin, 

 the chlorides of platinum and gold, etc. The formation 

 of a precipitate depends on the nature of the alkaloid, on 

 the acid in which it is dissolved, on the concentration, and 

 on the relative amounts of substance and reagent. As is 

 to be expected, there is a wide range in point of delicacy. 

 Phosphomolybdic acid, bismuth potassium iodide, and 

 tannin are most to be recommended. To apply these 

 tests a solution of a trace of the material should be pre- 

 pared in a drop of dilute sulphuric acid (1 in 50), and 

 placed on a glass on a black under-surface. A drop of the 

 precipitant is then brorfght into contact with it on a glass 

 rod, and the formation of an amorphous precipitate is 

 looked for. The precipitates given by phosphomolybdic 

 acid are pale to distinct yellow; by bismuth potassium 

 iodide, brown to red ; and by tannin, dirty white. 



