THE VOLATILE PAET OE PLANTS. 83 



which remains in insoluble combination with oxide of 

 copper. 



IHetapectic acid.— By too long boilinjj, by prolonged contact 

 with acids or alkalies, and by decay, the peetic and pectosic acids, as well 

 as pectin, are transformed into still another substance, -viz., meiapectic 

 acid, which, according to Fremy, is a very soluble body of quite sour 

 taste. It is the last product of the transformation of the bodies of this 

 group with which we are acquainted. It exists, according- to Fremy, in 

 beet-molasses and decayed fruits. 



Exp. 35. — Stew a handful of sound cranberries, covered with water, 

 just long enough to make them soft. Observe the speedy solution of 

 the firm pectose. Strain through muslin. The juice contains soluble 

 pectin, which may be precipitated from a small portion by alcohol. 

 Keep the remaining juice heated to near the boiling point in a water 

 bath, (i. e., by immersing the vessel containing it in a larger one of boil- 

 ing water.) After a time, which is variable according to the condition of 

 the fruit, and must be ascertained by trial, the juice on cooling or stand- 

 ing solidifies to a jelly, that dissolves on warming, and reappears again 

 on cooling — Fremy's pectosic acid. By further heating, the juice may 

 form a jelly which is permanent when hot — peetic acid — and on still 

 longer exposure to the same temperature, this jelly may dissolve again, 

 by passing into Fremy's metapectic acid, which alcohol does not precip- 

 itate. 



Other ripe fruits, as quinces, stnawberries, peaches, grapes, apples, etc., 

 may be employed for this experiment, but in any case the time required 

 for the juice to run through these changes cannot be predicted safely, 

 and the student may easily fail in attempting to follow them. 



Chemical composition of the Pectose group. — O ur knowl- 

 edge on this point is very imperfect. Pectose itself, hav- 

 ing never been obtained pure, has not been analysed. The 

 other bodies of this group have been examined, but, owing 

 to the difficulty of obtaining them, in a state of purity, the 

 res,ults of diiferent observers are discordant. 

 The forfnulse of Febmt are as follows : 



Pectose, 



Pectin, 



Pectosic acid, 



Peetic acid, 



Metapectic acid, 



Grouven, {2ter Salzmnnder Bericht, p. 470,) has prepar- 

 ed pectin on the large scale from beet-root cake, (remaining 

 after the juice was expressed for sugar manufacture,) by 



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