ound them in different ways.

How difficult it is, however, to carry out such operations with the
greatest accuracy, can only be unknown to one who either has never
undertaken this occupation, or at least has not done so with sufficient
attention.

+2.+ Hitherto chemical investigators are not agreed as to how many
elements or fundamental materials compose all substances. In fact this
is one of the most difficult problems; some indeed hold that there
remains no further hope of searching out the elements of substances.
Poor comfort for those who feel their greatest pleasure in the
investigation of natural things! Far is he mistaken, who endeavours to
confine chemistry, this noble science, within such narrow bounds! Others
believe that earth and phlogiston are the things from which all material
nature has derived its origin. The majority seem completely attached to
the peripatetic elements.

+3.+ I must admit that I have bestowed no little trouble upon this
matter in order to obtain a clear conception of it. One may reasonably
be amazed at the numerous ideas and conjectures which authors have
recorded on the subject, especially when they give a decision respecting
the fiery phenomenon; and this very matter was of the greatest
importance to me. I perceived the necessity of a knowledge of fire,
because without this it is not possible to make any experiment; and
without fire and heat it is not possible to make use of the action of
any solvent. I began accordingly to put aside all explanations of fire;
I undertook a multitude of experiments in order to fathom this beautiful
phenomenon as fully as possible. I soon found, however, that one could
not form any true judgment regarding the phenomena which fire presents,
without a knowledge of the air. I saw, after carrying out a series of
experiments, that air really enters into the mixture of fire, and with
it forms a constituent of flame and of sparks. I learned accordingly
that a treatise like this, on fire, could not be drawn up with proper
completeness without taking the air also into consideration.

[Footnote A: Carl Wilhelm Scheele's Chemische Abhandlung von der Luft
und dem Feuer. Upsala and Leipzig, 1777.]

+4.+ Air is that fluid invisible substance which we continually breathe,
which surrounds the whole surface of the earth, is very elastic, and
possesses weight. It is always filled with an astonishing quantity of
all kinds of exhalations, which are so finely su