r many purposes
for which it is adapted, such as a solvent in the preparation
of paints and varnishes and as a material for the preparation
of many important organic compounds. By an act of Congress in
1906, the tax was removed from _denatured_ alcohol, that is
alcohol mixed with some substance which renders it unfit for
the purposes of a beverage but will not impair its use for
manufacturing purposes. Some of the European countries have
similar laws. The substances ordinarily used to denature
alcohol are wood alcohol and pyridine, the latter compound
having a very offensive odor.

~Fermentation.~ The reaction which takes place in the preparation
of ethyl alcohol belongs to the class of changes known under
the general name of fermentation. Thus we say that the yeast
causes the glucose to ferment, and the process is known as
alcoholic fermentation. There are many kinds of fermentations,
and each is thought to be due to the presence of a definite
substance known as an _enzyme_, which acts by catalysis. In
many cases, as in alcoholic fermentation, the change is brought
about by the action of minute forms of life. These probably
secrete the enzymes which cause the fermentation to take place.
Thus the yeast plant is supposed to bring about alcoholic
fermentation by secreting the enzyme known as zymase.

~Glycerin~ (C_{3}H_{5}(OH)_{3}). This compound may be regarded as derived
from propane (C_{3}H_{8}) by displacing three atoms of hydrogen by three
hydroxyl groups, and must therefore be regarded as an alcohol. It is
formed in the manufacture of soaps, as will be explained later. It is an
oily, colorless liquid having a sweetish taste. It is used in medicine
and in the manufacture of the explosives nitroglycerin and dynamite.

ALDEHYDES

When alcohols are treated with certain oxidizing agents two hydrogen
atoms are removed from each molecule of the alcohol. The resulting
compounds are known as aldehydes. The relation of the aldehydes derived
from methyl and ethyl alcohol to the alcohols themselves may be shown as
follows:

Alcohols {CH_{3}OH Corresponding aldehydes {CH_{2}O
{C_{2}H_{5}OH {C_{2}H_{4}O

The first of these (CH_{2}O) is a gas known as formaldehyde. Its aqueous
solution is largely used as an antiseptic and disinfectant under the
name of _f