192 PHYSIOLOGY. [VIII. 



converts pectin into pectic acid, which is insoluble in cold water. 

 Continued ebullition of pectic acid in water converts it into 

 soluble parapectic acid, and this is easily changed by dilute 

 acids into nietapectic acid. The last" acid is also formed by 

 the action of strong acids on pectin, or of an excess of alkali 

 on pectin, pectosic or pectic acid. The following table shows 

 the composition of these bodies and their respective compounds 

 with oxide of lead. 



Lead-salt. 



Pectose 



Pectin 8HO,Cb,H,A6 



Parapectin 8HO,Cg,H,oO,g 7H0, PbO,Cg,H^03e 



Metapectin 8HO,Cg,H,o03g 6H0, 2VhO,Q^Jl^,0^^ 



Pectosic acid.... 3H0,C33H^0,3 HO, 2PbO,C3,H,„0,, 



Pectic acid 2HO,03,H^O,, 2PbO,C3,H^O,3 



Parapectic acid 2HO,a,H,30,, 2PbO,C,,H,30,, 



Metapectic acid 2H0,C3 H^ 0, • 2PbO,C3 H^ 0, 



The above series commences with neutral pectin, and passes 

 through a series of bodies successively more acid, to a strong 

 acid, the metapectic. They either differ from each other by 

 the elements of water, or are isomeric. 



The changes of the pectin series by water, acids and alka- 

 lies are similar to those which take place in the ripening of 

 fruits. Unripe fruits contain pectose, which is, during ripening, 

 gradually converted into pectin and parapectin, by the action 

 of acids present (malic, &c.) ; and these are changed by pec- 

 tas into metapectic acid, which unites with potassa or lime. 

 The metapectic acid probably causes the conversion of starch 

 into sugar. Boiling unripe fruits induces a similar formation 

 of pectin, which by the action of pectas is transformed into 

 gelatinous pectosic and pectic acids, forming a jelly, (xinn. d. 

 Ch. u. Pharm. Ixvii.) 



2. The Atmosphere. — The presence of carbonic acid in the 

 air has long been known. Its influence on vegetation has 

 been brought out more prominently by Liebig. Many experi- 

 ments seem to determine the presence of ammonia in the air. 



