ON COLLOID CHEMISTRY AND ITS INDUSTRIAL APPLICATIONS. 79 
in cold water, but quite soluble in dilute acids. On boiling with water 
it dissolves, but reverts to a gel on cooling. It is very quickly converted 
into pectic acid by boiling with water, by pectase, and by alkalies in 
excess. The pectosates form amorphous gels (Pelouze et Fremy). Pectic 
acid is formed by the action of alkalies upon pectin, also by the action 
of HCl upon beetroot pulp; it dissolves in boiling water, but reverts 
to a gel on cooling (Pelouze et Fremy). By long-continued boiling with 
water or by acids it is converted into parapectic acid ; alkalies convert 
it into metapectic acid. Parapectic acid dissolves in cold water, and 
the solution on evaporation leaves a residue which resembles gum or 
gelatin. The solution reddens litmus and is precipitated on addition of 
a mineral acid, acetic acid, or Ba(OH),. It yields a gelatinous precipitate 
with alcohol. Metapectic acid is stated to be identical with arabic acid. 
It is soluble in water, forming turbid or heterogeneous sols, yields a barium 
salt and is precipitated by alcohol, but not completely by basic lead 
acetate in excess (J. Weisberg, ‘Chem. Zeit.’ 18, 2). 
We have therefore the following series of products : 
Pectose Metapectin Parapectic acid 
Pectin Pectosic acid Metapectie acid 
Parapectin Pectic acid 
The ultimate analyses of these products by various observers do not 
agree among themselves, therefore we do not know how many individual 
members really exist; one fact, however, is clear, that is, that they contain 
more oxygen and less hydrogen than the true carbohydrates and that 
they have for the most part acid functions which become more marked 
by partial hydrolysis, showing that they contain one or more COOH 
groupings. According to later investigations (E. Bourquelot, ‘ Comptes 
Rend.’ 128, 1241-1244, and Bourquelot and Herissy, ‘Jour. Pharm. 
Chem.’ 1899, 9, 281-286), the pectens so far examined yield galactose 
and arabinose on hydrolysis, so that they probably contain galactan 
and araban. Cross and Bevan (‘ Cellulose,’ p. 217) state that ‘the pectic 
group of compounds may be regarded as compounds of carbohydrates 
of varied constitution with acid groups of undetermined constitution 
associated to form molecular complexes, more or less homogeneous, but 
entirely resolved by the continued action of simple hydrolytic agency.’ 
According to 8. B. Schryver and D. Haynes (‘ Biochem. Jour.’ 1906, 
10, 539-547), ‘ pectinogen ’ is obtained by treating turnips with a solution 
of ammonium oxalate and precipitating with alcohol. It has acid func- 
tions, and when kept in alkaline solutions it readily reverts to pectin, 
which also has acid characters. It is soluble in alkalies and is precipi- 
tated as a gel on addition of acids. Analysis shows it to have the com- 
position C,,H.,0,5. It yields furfural equivalent to one atom of carbon 
in each complex of C,;. Pectin would therefore appear to be an acid 
and not a carbohydrate. 
Pectins are more particularly interesting since they form the basis 
of fruit-jellies, such fruits as currants, gooseberries, and apples being 
especially rich in these constituents. The mucilages of linseed, marsh- 
mallows, orchids, and other plants are also worth attention. 
Several colloidal products from plants have been proposed for use 
from time to time, but very few have been found sufficiently valuable to 
