ients, it is found that the quantity
is also influential, the degree of heat evolved entering as a
factor into the result. Before we can apply the principle of single
difference, we must make sure that there is really only a single
difference between the instances that we bring into comparison.

The air-pump was invented shortly before the foundation of the Royal
Society, and its members made many experiments with this new means
of isolating an agent and thus discovering its potentialities.
For example, live animals were put into the receiver, and the air
exhausted, with the result that they quickly died. The absence of the
air being the sole difference, it was thus proved to be indispensable
to life. But air is a composite agent, and when means were contrived
of separating its components, the effects of oxygen alone and of
carbonic acid alone were experimentally determined.

A good example of the difficulty of excluding agencies other than
those we are observing, of making sure that none such intrude,
is found in the experiments that have been made in connexion with
spontaneous generation. The question to be decided is whether life
ever comes into existence without the antecedent presence of living
germs. And the method of determining this is to exclude all germs
rigorously from a compound of inorganic matter, and observe whether
life ever appears. If we could make sure in any one case that no germs
were antecedently present, we should have proved that in that case at
least life was spontaneously generated.

The difficulty here arises from the subtlety of the agent under
observation. The notion that maggots are spontaneously generated in
putrid meat, was comparatively easy to explode. It was found that when
flies were excluded by fine wire-gauze, the maggots did not appear.
But in the case of microscopic organisms proof is not so easy. The
germs are invisible, and it is difficult to make certain of their
exclusion. A French experimenter, Pouchet, thought he had obtained
indubitable cases of spontaneous generation. He took infusions of
vegetable matter, boiled them to a pitch sufficient to destroy all
germs of life, and hermetically sealed up the liquid in glass
flasks. After an interval, micro-organisms appeared. Doubts as to the
conclusion that they had been spontaneously generated turned upon two
questions: whether all germs in the liquid had been destroyed by the
preliminary boiling, and whether germs could have