82 HOW CROPS GROW. 
pectose, are subjected to the joint action of a moderate 
heat and an acid, the starch they contain is slowly altered 
into dextrin and sugar, while the firm pectose shortly soft- 
ens, becomes soluble in water, and is converted into pec- 
tin. It is precisely these changes which occur in the bak- 
ing of apples and pears, and in the boiling of turnips, car- 
rots, etc., with water. In the ripening of fruits the same 
transformation takes place. The firm pectose, under the 
influence of the acids that exist in all fruits, gradually soft- 
ens, and passes into pectin. 
Exp. 33.—Express, and, if turbid, filter through muslin the j juice of a 
ripe apple, pear, or peach. Add to the clear liquid its own bulk of al- 
cohol. Pectin is precipitated as a stringy, gelatinous mass, which, on 
drying, shrinks greatly in bulk, and forms, if pure,a white substance 
that may be easily reduced to powder, and is readily soluble in cold 
water. 
Exp. 34.—Reduce several white turnips or beets to pulp by 7 grating. 
Inclose, the pulp in a piece of muslin, and wash by squeezing in water 
until all soluble matters are removed, or until the water comes off nearly 
tasteless. Bring the washed pulp into a glass vessel, with enough dilute 
ellorhydric acid, (1 part by bulk of commercial muriatic acid to 15 
parts of water,) to saturate the mass, and let it stand 48 hours. Squeeze 
out the acid liquid, filter it, and add alcohol, when pectin will separate. 
The strong aqueous solution of pectin iseviscid or gummy, 
as seen in the juice that exudes from baked apples or pears. 
Pectosic and Pectic acids,—Under the action of a fer- 
ment occurring in many fruits, assisted by a gentle heat, 
pectin is transformed first into pectosic, and afterward into 
pectic acid. These bodies compose the well-known fruit- 
jellies. They are both insoluble in cold water, and remain 
suspended in it as a gelatinous mass. Pectosic acid is 
soluble in boiling water, and hence most fruit jellies be- 
come liquid when heated to boiling; on cooling, its solu- 
tion gelatinizes again. Pectic acid is insoluble even in 
boiling water. It is formed also when the pulp of fruits 
or roots containing pectose is acted on by alkalies or by 
ammonia-oxide of copper. The latter agent, (a solvent 
of cellulose,) converts pectose directly into pectic acid, 
