Still other difficulties show themselves in such investigations. It
seems very easy to collect carbonic acid in potash tubes, and to
determine its amount from the increase in weight of the tubes; but,
alas! to how many sources of error is this method exposed. If the potash
has been in contact with any organic substance, it will absorb oxygen.
If the pumice that takes the place of the potash contains protoxide of
iron, it will also absorb oxygen. In both cases the oxygen increases the
weight of the carbonic acid.
Every experimenter who has been compelled to repeat the weighing of a
somewhat complicate piece of apparatus, with an interval of several
hours between, knows how many inaccuracies he is exposed to if he is
compelled to take into calculation the changes of temperature and
pressure, and the moisture on the surface of the apparatus. After
fighting all these difficulties, and frequently in vain, the
experimenter begins to mistrust every result that depends only on
difference in weight, and to prefer those methods whereby the substance
to be estimated can be isolated, so that it can be seen and handled,
weighed or measured, in a free state, and in its own natural condition.
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