In complex apparatus
the exact office of each part should be understood.
3. A clear understanding of what happens. To this I have already
referred.
4. Why it happens.
5. In what other way it might be made to happen. In chemistry almost
every substance can be prepared in several different ways. The common
method is in most cases made so by some consideration of convenience,
cheapness, or safety. Often only one method is considered in one place
in a text book. In a review, however, several methods can be associated
together. Tests, uses, etc., will vary, too, and should be studied with
that fact in view. In physics phenomena illustrating a given principle
can usually be made to take place in several different ways. Often very
simple apparatus will do to illustrate some fact for which complex and
costly apparatus would be convenient. In such case the study of the
experiment with that fact in view becomes important to us who need to
simplify apparatus as much as possible.
6. Special precautions which may be necessary. Some experiments always
work well, even in the hands of those not used to the work. Others are
successful--sometimes safe, even--only when the greatest care is taken.
Substances are used constantly in work in chemistry which are deadly
poisons, others which are gaseous and will pass through the smallest
holes. In physics the experiments usually present fewer difficulties of
this sort.
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