e. A century or two after the time of
Mohammed, the Arabians of his faith, a people who had acquired Greek
science from the libraries which their conquests gave them, conducted
extensive experiments, and named a good many familiar chemical
products, such as alcohol, which still bears its Arabic name.

These chemical studies were continued in Europe by the alchemists, a
name also of Arabic origin, a set of inquirers who were to a great
extent drawn away from scientific studies by vain though unending
efforts to change the baser metals into gold and silver, as well as to
find a compound which would make men immortal in the body. By the
invention of the accurate balance, and by patient weighing of the
matters which they submitted to experiment, by the invention of
hypotheses or guesses at truth, which were carefully tested by
experiment, the majestic science of modern chemistry has come forth
from the confused and mystical studies of the alchemists. We have
learned to know that there are seventy or more primitive or apparently
unchangeable elements which make up the mass of this world, and
probably constitute all the celestial spheres, and that these elements
in the form of their separate atoms may group themselves in almost
inconceivably varied combinations. In the inanimate realm these
associations, composed of the atoms of the different substances,
forming what are termed molecules, are generally composed of but few
units. Thus carbonic-acid gas, as it is commonly called, is made up of
an aggregation of molecules, each composed of one atom of carbon and
two of oxygen; water, of two atoms of hydrogen and one of oxygen;
ordinary iron oxide, of two atoms of iron and three of oxygen. In the
realm of organic life, however, these combinations become vastly more
complicated, and with each of them the properties of the substance
thus produced differ from all others. A distinguished chemist has
estimated that in one group of chemical compounds, that of carbon, it
would be possible to make such an array of substances that it would
require a library of many thousand ordinary volumes to contain their
names alone.

It is characteristic of chemical science that it takes account of
actions which are almost entirely invisible. No contrivances have been
or are likely to be invented which will show the observer what takes
place when the atoms of any substance depart from their previous
combination and enter on new arrangements. We only