128 LECTURES ON 



From the best analyses, and from analogy with cellulose, it is prob- 

 able that pectose has the same composition as pectin, or, like the 

 pectic and pectosic acids, differs from it only by one or more equiva- 

 lents of water. This relatedness of composition assists us here, as in 

 case of the preceding groups of organic principles, to comprehend, in 

 some measure, the ease with which the transformations of these bodies 

 are effected. 



It will be perceived, by a glance at the composition of the pectose 

 group, that their oxygen exceeds the quantity necessary to form water 

 with their hydrogens by eight equivalents. 



The vegetal acids are exceedingly numerous. They are found in all 

 classes of plants, and nearly every family in the vegetable kingdom 

 has one or more acids peculiar to itself. 



Those we shall now notice are few in number, but of almost uni- 

 versal distribution. They are oxalic, tartaric, citric, and malic acids. 

 In plants they never occur in the free or pur« state, but always com- 

 bined with lime, potash, ammonia, &c. They are most often accumu- 

 lated in large quantity in fruits. 



Oxalic acid exists largely in the common sorrel, and, according to 

 the best observers, is found in greater or less quantity in nearly all 

 plants. The pure acid presents itself in the form of colorless bril- 

 liant transparent crystals not unlike Epsom salts in appearance, but 

 having an intensely sour taste. It is prepared for commerce by sub- 

 jecting starch or cane and grape sugar to rapid oxydation, generally 

 by means of nitric acid. Salt of sorrel, employed to remove ink- 

 stains from cloth and leather, is an oxalate of potash and water. 



Tartaric acid is especially abundant in the grape, from the juice of 

 which during fermentation it is deposited in combination with potash, 

 as anjol, which, by purification, yields the cream of tartar of com- 

 merce. Tartaric acid, when pure, occurs in large glass} r crystals very 

 sour to the taste. It has recently been observed by Liebig as one of 

 the products of the artificial oxydation by nitric acid, of the peculiar 

 sugar found in milk, and is also probably a result of the oxydation of 

 gum by the same reagent. 



Malic acid is the chief sour principle of apples, currants, gooseber- 

 ries, and many other fruits. It exists in large quantity in the garden 

 rhubarb, in the berries of the mountain ash and barberry, and in the 

 leaves of the beet and tobacco plants. 



Citric acid is most abundant in the juice of the lemon, lime, and 

 cranberry. 



All these acids usually occur together in our ordinary fruits, and in 

 some cases it is certain that they are converted the one into another 

 during the development of the plant. 



Their composition is expressed in the following table: 



Oxalic acid C 4 6 -f 2 HO. 



Malic acid C s H 4 8 + 2 HO. 



Tartaric acid C s H 4 0^+2 HO. 



Citric acid C 12 H 5 O u + 3 HO. 



The vegetal acids exert an important influence in the plant as 



