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

OR SCIENTIFIC METHOD. 281
strewed with large naked stones, and worm-castings were
almost wholly absent. Obscure lines and irregularities on
the surface indicated that the land had been cultivated some
centuries ago. It is probable that a thick wood of young
beech-trees sprang up so quickly, that time enough was not
allowed for worms to cover up the stones with their castings,
before the site became unfitted for their existence."
Hence we have good grounds for believing that earth
worms are the agency by which vegetable mould is formed,
and that it is formed by no other means.
7. The nature of the Double Method of Difference
may be thus expressed : When
one phenomenon has
been shown to be the cause of another under given
conditions, by the Method of Single Difference; and
when we fail to find any instance where the second
phenomenon
probable
"
antecedent
has occurred without the first : then it is
that the first is the
"
unconditionally invariable
of the second i.e., that the latter can be
produced in no other way than by the former ;
and the
probability increases with the number and variety of the
negative instances. The Method presupposes
that of
Single Difference, and goes beyond it in examining the
negative instances independently. Very often, perse
vering experiments are necessary in testing various pos
sible negative instances.
The extent of the field over which we must range in
which the
assembling negative instances is a question
trained investigator, possessing wide and accurate know
ledge of the subject, alone can decide. It depends on
the kind of problem in question, and the advanced state
(or the reverse) of the science to which it belongs. In
Chemistry,
there is reason to believe that we have ex
perimental knowledge of nearly all the elements to be
found on earth. Hence, when by the Method of Single
Difference an element yields a particular reaction (i.e.,