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

288 THE THEORY OF INDUCTION
of Uranus in its elliptic orbit ; but there were residual
perturbations which could not be thus accounted for.
From these the orbit and position of Neptune were
calculated before the planet had been observed.
Mill refers to the Method of Residues as available
when special difficulties arise in observation, because
several causes act at once, and their effects are all
blended together, producing a joint effect of the same
kind as the separate effects (Logic, Bk. III. x. 3
and 4). Mill s view of what he calls intermixture of
effects has been simply explained by Jevons :
"
If in
one experiment friction, combustion, compression, and
electric action are all going on at once, each of these
causes will produce quantities of heat which will be
added together, and it will be difficult or impossible to
say how much is due to each cause separately. We
may call this a case of the homogeneous intermixture
of effects, the name indicating that the joint effect is of
the same kind as the separate effects. It is distinguished
by Mill from cases of the heterogeneous, or, as he says,
the heteropathic intermixture of effects, where the joint
effect is totally different in kind from the separate effects.
Thus if we bend a bow too much it breaks instead of
bending farther ;
if we warm ice it soon ceases to rise
in temperature and melts ; if we warm water it rises in
temperature homogeneously for a time, but then sud
denly ceases, and an effect of a totally different kind, the
production of vapour, or possibly an explosion, follows.
Now, when the joint effect is of a heterogeneous kind,
the method of difference is sufficient to ascertain the
cause of its occurrence. Whether a bow or a spring
will break with a given weight may easily be tried, and
whether water will boil at a given temperature in any
given state of the barometer may also be easily ascer-