carbonic acid gas.

During the same time it gradually became evident that the presence of
sugar was essential to the production of alcohol and the evolution of
carbonic acid gas, which are the two great and conspicuous products of
fermentation. And finally, in 1787, the Italian chemist, Fabroni, made
the capital discovery that the yeast ferment, the presence of which is
necessary to fermentation, is what he termed a "vegeto-animal" substance;
that is, a body which gives of ammoniacal salts when it is burned, and
is, in other ways, similar to the gluten of plants and the albumen and
casein of animals.

These discoveries prepared the way for the illustrious Frenchman,
Lavoisier, who first approached the problem of fermentation with a
complete conception of the nature of the work to be done. The words in
which he expresses this conception, in the treatise on elementary
chemistry to which reference has already been made, mark the year 1789 as
the commencement of a revolution of not less moment in the world of
science than that which simultaneously burst over the political world,
and soon engulfed Lavoisier himself in one of its mad eddies.

"We may lay it down as an incontestable axiom that, in all the operations
of art and nature, nothing is created; an equal quantity of matter exists
both before, and after the experiment: the quality and quantity of the
elements remain precisely the same, and nothing takes place beyond
changes and modifications in the combinations of these elements. Upon
this principle the whole art of performing chemical experiments depends;
we must always suppose an exact equality between the elements of the body
examined and those of the products of its analysis.