etween a stone slab and roller called a _metate_, then patted and
tossed from hand to hand until flattened into thin, wafer-like cakes,
and baked over a quick fire, on a thin iron plate or a flat stone.

Unquestionably, unleavened bread, well kneaded and properly baked, is
the most wholesome of all breads, but harder to masticate than that made
light by fermentation, but this is an advantage; for it insures more
thorough mixing with that important digestive agent, the saliva, than is
usually given to more easily softened food.

[Illustration: Stone Metate.]

What is usually termed unfermented bread, however, is prepared with
flour and liquid, to which shortening--of some kind is added, and the
whole made light by the liberation of gas generated within the dough
during the process of baking. This is brought about either by mixing
with the flour certain chemical substances, which, when wet and brought
into contact, act upon each other so as to set free carbonic acid gas,
which expands and puffs up the loaf; or by introducing into the dough
some volatile substance as carbonate of ammonia, which the heat during
baking will, cause to vaporize, and which in rising produces the same
result.

Carbonic acid gas maybe for this purpose developed by the chemical
decomposition of bicarbonate of potassa (saleratus), or bicarbonate of
soda, by some acid such as sour milk, hydrochloric acid, tartaric acid,
nitrate of potassa, or the acid phosphate of lime.

The chemical process of bread-raising originally consisted in adding to
the dough definite proportions of muriatic acid and carbonate of soda,
by the union of which carbonic acid gas and common salt were produced.
This process was soon abandoned, however, on account of the propensity
exhibited by the acid for eating holes in the fingers of the baker as
well as in his bread pans; and a more convenient one for hands and
pans, that of using soda or salaratus with cream of tartar or sour milk,
was substituted. When there is an excess of soda, a portion of it
remains in the loaf uncombined, giving to the bread a yellow color and
an alkaline taste, and doing mischief to the delicate coating of the
stomach. Alkalies, the class of chemicals to which soda and salaratus
belong, when pure and strong, are powerful corrosive poisons. The acid
used with the alkali to liberate the carbonic-acid gas in the process of
bread-making, if rightly proportioned, destroys this poisonous property,
and