44 PROXIMATE CONSTITUENTS OF PLANTS 



The following tests for hydrocyanic acid are then 

 applied to the distillate : — 



(i.) A silver nitrate solution produces a white 

 precipitate of AgCN. 



(ii.) A little of the solution is placed in a basin 

 together with a few drops of yellow ammonium sulphide, 

 and the mixture evaporated nearly to dryness. A 

 thiocyanate (sulphocyanide) is produced as shown by 

 the addition of a drop of ferric chloride solution, when 

 the characteristic blood-red coloration of ferric thio- 

 cyanate is formed. 



(iii.) A little caustic soda solution is added, then a 

 drop or two each of ferric chloride and ferrous sulphate 

 solutions. On warming and acidifying with hydrochloric 

 acid, a dark blue precipitate of Prussian blue (ferric 

 ferrocyanide) is produced. 



By the same method as that described above, the presence of 

 amygdalin can be shown in cherry -laurel leaves, and in the kernels 

 of peach, plum, and cherry stones. 



{g) The Organic Acids. 



Various organic acids — formic, oxalic, malic, citric, tartaric, etc. 

 — occur very widely distributed in the vegetable kingdom, and are 

 hence sometimes known as the vegetable acids. 



They are generally present in the form of metallic salts, 

 especially those of potassium or calcium, but they sometimes occur 

 in the free state, as for example in unripe fruits, which owe their 

 acidity to this cause, and also to the presence of acid salts of 

 potassium and calcium. 



Some of the commoner organic acids and their salts occurring 

 in plants are enumerated : — 



Formic acidy H . COOH, in nettles. 



Add potassium oxalate^ COOH . COOK, in sorrel, rhubarb. 



Malic acid, COOH . CHOH . CHg. COOH, in apples, mountain- 

 ash berries, gooseberries, morella cherries. 



Acid potassium malate, COOK . CHOH . CH2 . COOH, in sweet 

 cherries. 



