3 22 PLANT AND ORGANIC CHEMISTRY 
the sugar-cane or beet-root, and milk sugar. Starches and 
gums, though conveying little idea of sugar, are chemically 
to be considered as sugars. 
The characteristics of these different compounds are very 
unlike. They vary from very soluble to insoluble compounds, 
and from crystalline to non-crystalline bodies. But the in¬ 
soluble compounds, like starch and cellulose, may be con¬ 
verted into the soluble sugars by the action of heat and dilute 
acids, and by certain ferments, as diastase. The reaction 
which accompanies this conversion involves the taking up of 
water, and at the same time the complicated molecule splits 
into several simpler ones. This reaction is called hydrolosis: 
x(C 6 H 10 O 5 ) + xH 2 0 = xC 6 H 12 0 6 . 
As will be observed, the sugar group — collectively desig¬ 
nated as “carbohydrates” — comprehends a vast widening- 
out vista of compounds, from a simple compound derived 
directly from the elements, to complex bodies with numerous 
isomers. 
The sugars of physiological consequence are widely spread 
in animals and plants, and, as carbohydrates, constitute one 
of the three great classes of natural organic compounds, the 
fats and albuminoids constituting the other two classes. La¬ 
voisier discovered that the materials of which carbohydrates 
are composed were carbon, hydrogen, and oxygen; but the 
objection to the use of the term carbohydrate, which is defined 
as a compound containing carbon and hydrogen and oxygen 
in the proportion of 2 to i, is its non-universality. The sugar 
called rhamnose, C 6 H 12 0 5 , may be mentioned as an excep¬ 
tion to the definition, but for purposes of classification the 
name carbohydrate has been retained by writers. 
The carbohydrates have been divided for convenience into 
three groups: 
(1) Simple sugars, or monosaccharides, as grape or fruit 
sugars. 
(2) Decomposable sugars, or polysaccharides, as cane or 
milk sugar and raffinose. 
(3) Polysaccharides unlike sugar, as starch, cellulose, and 
dextrine. 
