ol. Soon the thermometer in the tube along with radium reads
54 deg. F. higher than the other one. The tube without the radium, whose
temperature remains unaltered, is called the "control" experiment. Most
experiments are of the type (b); and since the Canon, which describes
two co-existing instances, does not readily apply to this type, an
alternative version may be offered: _Any agent whose introduction into
known circumstances (without further change) is immediately followed by
a definite phenomenon is a condition of the occurrence of that
phenomenon._

The words _into known circumstances_ are necessary to emphasise what is
required by this Method, namely, that the two instances differ in only
one thing; for this cannot be ascertained unless all the other
conditions are known; and this further implies that they have been
prepared. It is, therefore, not true (as Sigwart asserts) that this
method determines only one condition of a phenomenon, and that it is
then necessary to inquire into the other conditions. If they were not
known they must be investigated; but then the experiment would not have
been made upon this method. Practically, experiments have to be made in
all degrees of imperfection, and the less perfect they are, that is, the
less the circumstances are known beforehand, the more remains to be
done. A common imperfection is delay, or the occurrence of a latent
period between the introduction of an agent and the manifestation of its
effects; it cannot then be the unconditional cause; though it may be an
indispensable remote condition of whatever change occurs. If, feeling
out of sorts, you take a drug and some time afterwards feel better, it
is not clear on this ground alone that the drug was the cause of
recovery, for other curative processes may have been active
meanwhile--food, or sleep, or exercise.

Any book of Physics or of Chemistry will furnish scores of examples of
the method of Difference: such as Galileo's experiment to show that air
has weight, by first weighing a vessel filled with ordinary air, and
then filling it with condensed air and weighing it again; when the
increased weight can only be due to the greater quantity of air
contained. The melting-point of solids is determined by heating them
until they do melt (as silver at 1000 deg. C., gold at 1250 deg., platinum at
2000 deg.); for the only difference between bodies at the time of melting
and just before is the addition of so much heat. S