CLASSES OF ACIDS 189 



and blisters the skin. Vinegar usually has only 3% or 

 4% of it. 



The materials present in plants undergo other fermenta- 

 tions besides the one that gives acetic acid. Thus, "Dill " 

 pickles are small cucumbers fermented so as to give 

 lactic acid; " sauer kraut " contains the same acid. Sweet 

 milk becomes sour because the milk sugar in it is changed 

 to lactic acid. Pure lactic acid is a thick liquid of very 

 sour taste. 



214. Classes of Acids. It is not only by fermentation 

 of fruit juices that acids are formed. The fruits them- 

 selves, as well as other parts of plants, contain acids. 

 Tomatoes, cherries, rhubarb, etc., are strongly acid. Lem- 

 ons and oranges contain much citric acid; grape juice, 

 much tartaric acid; apples contain malic acid. All of the 

 acids already named in this chapter are compounds of 

 carbon with hydrogen and oxygen (c/. 123). These 

 acids are called organic acids (c/. 3). There are many 

 acids that contain no carbon at all, but have some other 

 element in place of the carbon. These belong to the class 

 of inorganic, or mineral, acids. Thus, nitric acid contains 

 nitrogen, combined with hydrogen and oxygen; sulphuric 

 acid consists of sulphur, hydrogen, and oxygen; phos- 

 phoric acid contains phosphorus, hydrogen, and oxygen. 

 These three acids are all thick, colorless liquids. They 

 are commonly " diluted " with much water. 



All acids contain hydrogen, but not all have oxygen. Thus, hydro- 

 chloric acid (c/. 111) contains only hydrogen and chlorine. This is 

 the acid of the gastric juice (cf. 364). The gastric juice of man con- 

 tains about 0.22 of 1% of it; the adult dog has several times as much. 



