154 FERMENTATION. 



is very sour, and does not crystallize ; it forms 

 salts with many of the metallic oxides. 



Gallic acid. — Tliis acid is found in gall-nuts. 

 It crystallizes, and forms whitish crystals, of a sour 

 taste and peculiar smell. When gallic acid is put 

 into a solution containing iron, a black precipitate 

 appears. The base of ink is iron thus precipi- 

 tated. To produce good black ink, infuse one 

 pound of powdered gall-nuts for four hours, 

 without boiling, in common water, with six ounces 

 of gum-arabic, and six ounces of sulphate of iron. 

 With gold, gallic acid forms a brown precipitate j 

 with silver, a grey ; with mercury, an orange; with 

 copper, a brown ; and with lead, a wliite. 



Citric acid is procured from the juice of lemons 

 and other fruits. It is capable of crystallizing. 

 Its crystals are soluble in water, and very sour. It 

 forms citrates with the earths, alkalies, and metals. 

 It is much used in calico-printing. It is also used 

 for discharging spots of ink from linen. 



Benzoic acid is obtained from gum-benzoin, or 

 benjamin. It is a crystallizable acid. The com- 

 pounds which it forms are called benzoates, 



Kinic acid is found in Peruvian bark. 



FERMENTATION. 



If mucilagi-nous saccharine vegetable substances 

 be subjected to the action of water and heat, (from 

 60 to 70 tleg. Fahr.) they experience, in a very 

 short time, a very striking change. An internal 

 commotion takes place, the mass grows turbid, a 

 large quantity of air-bubbles, consisting of carbonic 

 acid gas, are disengaged, which, on account of the 

 viscidity of the matter in which they are inclosed, 

 form a stratum on the surface of the fluid, known 



