118 BIOLOGY OF PLANTS. 



first precipitated, the oxalate of lime (C^OSCaOSHO =82.74) is a 

 snow-white, floculent powder. 



Oxalate of potassa* is commonly called the tssential salt oj 

 lemons, and is used to remove spots and iron rust from linens. 



2. Citric acid (C^H2O''=G0) is found in many acidulous fruits, 

 such as limes, lemons and oranges. It is distinguished by form- 

 ing an insoluble salt with lime, and is used in preparing 

 lemon syrup. 



3. Tartaric acid {^^11^0^=66.24:) exists in acidulous fruits, 

 especially in the juices of the mulberry and grape, usually in 

 combination with lime or potassa. It is the substance used 

 with soda for an effervescing drink. Bitarirate of potassa is 

 foimd in old wine casks, and when purified, is called cream oJ 

 tartar, and is used in medicine. Tartar emetic is a compound 

 of tartaric acid with potassa and antimony, and is neutralized 

 by astringents, such as tea or Peruvian bai"k, in case too large 

 a dose is taken. 



White Rochelle salt is formed of potassa and soda, combined 

 with tartaric acid. 



3. Benzoic acid (C^^H503=123) exists in gum benzoin, in 

 the balsams of Peru, and some other vegetable substances. It 

 is distinguished by its volatility and aromatic odor ; although, 

 when perfectly pure, it has no smell. When sublimed, it forms 

 long, flat, prismatic nodules, having a beautiful satin lustre. 



4. Meconic acid (C'^H20'''== 100,84) is found in the poppy, in 

 combination with morphia, and crystalizes in white, transj)arent 

 scales. 



5. Gallic acid (C'''H305=85) is found in gall-nuts and in the 

 bark of trees. It forms ink by combining with the proto-sul- 

 phuret of iron. 



6. Tannic acid (CiSH^Oi^^Qi^) exists also in gall-nuts, in 

 tea and vegetable astringents, and is the cause of their astrin- 

 gency. With gelatine or glue it forms an insoluble compound, 

 which is the basis of leather ; hence its use in tanning hides. 

 Tannic and gallic acids possess the property of preserving 

 bodies from decay. 



7. Acetic acid (C^H303=51,48) exists in the sap of many 

 plants. It is well known under the name o{ vinegar. It forms 

 numerous salts with inorganic bases, such as acetates of lead {su- 



* When an acid combines with an alkali or metallic oxide, the 

 substance formed is called a salt, and the name of the acid changes its 

 termination into ate, or if the acid terminate in ous, into ite. 



