VEGETABLE ACIDS. 



105 



The constituent principles of citric acid are found in 

 the following proportions ; 



Carbon 33.811 



Hydrogen 6.330 



Oxygen 59.859 



Acetic acid exists ready formed in the sap of plants ; 

 it is sufficiently distinguished from all the other vegetable 

 acids by the peculiar property it possesses of forming 

 easily soluble salts with the earths and alkalies. 



When a plant or any other vegetable product is distilled, 

 not only the acetic acid which it contains is extracted 

 from it, but a great quantity of acid is formed by that de- 

 composition and disunion of the constituent principles, 

 which is produced by heat. The smoke which escapes 

 from our fire-places is only a confused mixture of water, 

 acetic acid, oil, carbonic acid, and carbon. 



The acid produced by combustion and distillation has 

 been known for a long time under the name of pyroligneous 

 acid : it was not suspected to be the same as vinegar. 



A vast quantity of this acid may be procured with great 

 ease by the new method of carbonizing wood in close 

 vessels : the acid thus procured is however combined 

 with oil, which gives it a dark brown color, and a disa- 

 greeable empyreumatic odor; but by a particular process 

 it may be freed from all foreign matter, and rendered 

 perfectly pure : to effect this, the acid must be saturated 

 with lime or an alkali; after which the oil must be car- 

 bonized by exposing the i;iew salt impregnated with it, to 

 a degree of heat sufficient to effisct that change ; the salt 

 is then to be decomposed by pouring upon it sulphuric 

 acid ; or, the same result may be obtained by decompos- 

 ing the acetate of lime by means of an alkaline sulphate : 

 in this case an exchange of bases takes place, and the 

 acetate treated with sulphuric acid furnishes a very pure 

 acid.* 



* Wood is distilled in a great iron retort, the bottom of which is 

 of cast iron, and the sides of thick sheet iron ; when it is filled with 

 wood the lid of it is carefully luted on with clay. 



For distillation the wood must be very dry and the sticks prepared 

 of equal thickness. Each retort will contain two ^'voies" ( = 106 

 cubic feet) of wood. The opening or flue by which the smoke es- 

 capes, is placed at a distance of some inches from the bottom of the 

 boiler or retort. The acid is carried by copper pipes into a vessel, in 

 which the water is constantly renewed : the acid and tar flow by a 

 cock into a close vessel. The inflammable gas passes through copper 



