It obviously follows that
Iron is useful for telegraphic purposes. (4)
Now if we take our letters thus,
the premises will assume the forms
| A = AB, | (1) |
|---|---|
| B = BC, | (2) |
| C = CD. | (3) |
For B in (1) we can substitute its equivalent in (2) obtaining, as before,
Substituting for C in this intermediate result its equivalent as given in (3), we obtain the complete conclusion
A = ABCD. (4)
The full interpretation is that Iron is iron, metal, good conductor of electricity, useful for telegraphic purposes, which is abridged in common language by the ellipsis of the circumstances which are not of immediate importance.
Instead of all the propositions being exactly of the same kind as in the last example, we may have a series of premises of various character; for instance,
| Common salt is sodium chloride, | (1) |
|---|---|
| Sodium chloride crystallizes in a cubical form, | (2) |
| What crystallizes in a cubical form does not possess the power of double refraction; | (3) |
it will follow that
Common salt does not possess the power of double refraction. (4)
Taking our letter-terms thus,
we may state the premises in the forms
| A = B, | (1) |
|---|---|
| B = BC, | (2) |
| C = C d . | (3) |
Substituting by (3) in (2) and then by (2) as thus altered in (1) we obtain
A = BC d , (4)
which is a more precise version of the common conclusion.
We often meet with a series of propositions describing the qualities or circumstances of the one same thing,