An introductory text-book of logic - Mellone, Sydney - Rare Books at ...

266 THE THEORY OF INDUCTION
or periodical returns of those bodies. Thus it is that
the meteorologist observes the ever-changing weather,
and notes the height of the barometer, the temperature
and moistness of the air, the direction and force of the
wind, the height and character of the clouds, without
being in the least able to govern any of these facts.
The geologist, again, is generally a simple observer when
he investigates the nature and position of rocks. The
zoologist, the botanist, and the mineralogist usually
employ mere observation when they examine animals,
plants, and minerals, as they are met with in their
natural condition.
"
In experiment, on the contrary, we vary at our will
the combinations of things and circumstances, and then
observe the result. It is thus that the chemist discovers
the composition of water by using an electric current to
separate its two constituents, oxygen and hydrogen. The
mineralogist may employ experiment when he melts two
or three substances together to ascertain how a particular
mineral may have been produced. Even the botanist and
zoologist are not confined to passive observation ; for by
removing animals or plants to different climates and
different soils, and by what is called domestication,
they may try how far the natural forms and species
are capable of alteration."
All the advantages of experiment spring from the fact
that by this means we can isolate or insulate an event by
presenting it under circumstances with which we are fully
acquainted. The importance of thus
"
circumstances
"
knowing
all the
will be seen in the following sections ;
meanwhile the following passage from Sir J. Herschel s
Discourse on the Study of Natural Philosophy will
help to point
experiment :
the contrast between observation and