150 



sill H. DAVY 3 AGRICULTURAL CHEMISTRV , 



body forms with the red oxide of iron 

 the deep bright blue substance called 

 Prussian blue. 



Two other vegetable acids have been 

 found in the products of plants ; the 

 morolyxac acid in a saline exudation from 

 the white mulberry tree, and the tinic 

 acid in a salt afforded by Peruvian bark; 

 but these two bodies have as yet been 

 discovered in no other cases. The phos- 

 phoric acid is found free in the onion ; 

 and the phosphoric, sulphuric, muriatic, 

 and nitric acids, exist in many saline com- 

 pounds in the vegetable kingdom ; but 

 they cannot with propriety be consider- 

 ed as vegetable products. Other acids 

 are produced during the combustion of 

 vegetable compounds, or by the action of 

 nitric acid upon them ; they are the 

 camphoric acid, the mucous or saclactic 

 acid, and the suberic acid ; the first of 

 which is procured from camphor ; the 

 second from gum or mucilage, and the 

 third from cork, by the action of nitric 

 acid. From the experiments that have 

 been made upon the vegetable acids, it 

 appears that all of them, except the prus- 

 sic acid, are constituted by different pro- 

 portions of carbon, hydrogen, and oxy- 

 gen ; the prussic acid consists of carbon, 

 azote and hydrogen with a little oxygen. 

 The gallic acid contains more carbon than 

 any of the other vegetable acids. 



The following estimates of the compo- 

 sition of some of the vegetable acids 

 have been made by Gay Lussac and 

 Thenard. 



100 parts of oxalic acid contain : 



Carbon 26.566 



Hydrogen - - - - 2.745 

 Oxygen ----- 70.689 

 100 parts of tartaric acid contain : 



Carbon 24.050 



Hydrogen ... - 6.629 



Oxygen 69.321 



100 parts of citric acid contain : 



Carbon 33.811 



Hydrogen - - - - 6.330 



Oxygen 59.859 



100 parts of acetic acid contain : 

 Carbon . . - - 50.224 



Hydrogen - - - - 5.620 

 Oxygen . . . . 44.147 



100 of mucous or saclactic acid con- 

 tain : 



Carbon - - - - . • 33.69 



Hydrogen . - . . 3,62 

 Oxygen - . - . 62.69 



These estimations agree nearly with the 

 following definite proportions. In oxalic 

 acid 7 proportions of carbon, 8 of hydro- 

 gen, and 15 of oxygen ;* in tartaric acid, 

 8 of carbon, 28 hydrogen, IS of oxygen ; 

 in citric acid, 3 carbon, 6 hydrogen, 4 

 oxygen ; in acetic acid, IS carbon, 22 

 hydrogen, 12 oxygen ; in mucous acid, 

 6 carbon, 7 hydrogen, 8 oxygen. 



The applications of the vegetable acids 

 are well known. The acetic and citric 

 acids are extensively used. The agreea- 

 ble taste and wholesomeness of various 

 vegetable substances used as food, mate- 

 rially depend upon the vegetable acid 

 they contain. 



19. Fixed alkali may be obtained in 

 aqueous solution from most plants by 

 burning them, and treating the ashes with 

 quick lime and water. The vegetable 

 alkali, or potassa, is the common alkali, 

 in the vegetable kingdom. 



This substance in its pure state is 

 white, and semi-transparent, requiring a 

 strong heat for its fusion and possessed 

 of a highly caustic taste. In the matter 

 usually called pure potassa by chemists, 

 it exists combined with water ; and in 

 that commonly called pearl-ashes or pot- 

 ashes in commerce, it is combined with 

 a small quantity of carbonic acid. Po- 

 tassa in its combined state, as has been 

 mentioned, page 94, consists of the high- 

 ly inflammable metal potassium, and oxy- 

 gen, one proportion of each. 



Soda, or the mineral alkali, is found in 

 some plants that grow near the sea, and 

 is obtained combined with water, or car- 

 bonic acid, in the same manner as potas- 

 sa ; and, consists, as has been stated, page 

 94, of one portion of sodium, and two 

 proportions of oxygen. In its properties 

 it is very similar to potassa ; but may be 

 easily distinguished from it by this cha- 

 racter ; it forms a hard soap with oil ; 

 potassa forms a soft soap. 



Pearl-ashes, and barilla, and kelp, or 

 the impure soda obtained from the ashes 



* According to Dr. Thompson's experiments, oxalic 

 acid contains of 3 proportions of carbon, 4 of oxygen, 

 and 4 of hydrogen, a result very diflerent, indeed, from 

 that of the French chemists. 



