§ 89. OCCUEKEXCE OF SUBSTANCES. 145 



C. 



For the mode of occurrence of the substances for whose 

 detection directions are given in the j^receding pages, in 

 agricultural materials and products, consult Table X, ex- 

 cept in case of the following, which are not widely dis- 

 tributed, or whose occurrence presents comparatively less 

 interest, because they have not been quantitatively esti- 

 mated in these materials or products. 



Acid, acetic, besides occurring in vinegar, which re- 

 sults from the action of the air on alcoholic liquors, is 

 found among the products of the putrefaction, or of the 

 destructive distillation of organic matter. 



Acid, citric, is found in lemons, and in most other acid 

 fruits^ such as gooseberries, cherries, etc. 



Acid, lactic, is the acid of sour milk, and is found also 

 in some animal juices, and sometimes in urine. 



Acid, malic, is found in unripe apples, and in most un- 

 ripe fruits, together with citric acid, and also in potatoes, 

 in many roots, and in the stems and leaves of many 

 plants, such as rhubarb, tobacco, etc. 



Acid, tartaric, is found, like malic acid, in many fruits, 

 and particularly in the grape ; it occurs also in the roots, 

 stems, and leaves, of many plants. 



Arsenic may be found occasionally in superphosphates, 

 where it was derived from the sulphuric acid used in the 

 manufacture of the article ; it is also a frequent and dan- 

 gerous ingredient of bright green pigments. 



Barium may sometimes be found as a silicate in some 

 common rocks, and hence in soils. 



Copper may sometimes be found in culinary products 

 where vessels made of the metal or its alloys have been 

 used ; it is a frequent and harmful ingredient of bright 

 green pickles 



7 



