In order to see more clearly the differentiation between relations that require the semicolon and those that require the colon, let us study these relations in two sentences properly punctuated with each of the two marks:
63. The only true equalizers in the world are books; the only treasure-house open to all comers is a library; the only wealth which will not decay is knowledge.
64. Here was a superintendent worth having: when he did not find good tools, he made them.
Why not use commas or periods in No. 63? Why not use a semicolon in No. 64?
Perhaps we have not made, and cannot make, the differentiation between the comma and the semicolon sufficiently definite to answer the first question, especially as the formation of each of the three clauses is such that the liability to error in reading them is very slight. It would be rather dogmatic to assert that the use of commas here would be poor punctuation; yet the degree of separation between the groups is so marked as to make the relations more like the relations between groups that we have seen to require a semicolon than like relations between groups separated by a comma. The degree of separation here almost approaches the degree that takes the period; and therefore the semicolon seems much better than the comma.
But why not periods? Two positive answers may be given. The three clauses constitute one complete idea, and therefore make a sentence. If set off by periods, thus making three sentences, no fourth sentence could be formed to follow the third and maintain sentence unity; for the fourth sentence would necessarily grow out of the third, while its logical relation would be to the three clauses. Such a fault would be that seen in Sentence 62.
Why not a semicolon in No. 64? Because, under our classifications and definitions, the colon, when its use is not determined by its rank, as it is in Sentence 6, is used to show a relation that is practically that of apposition, exhibited, in different forms, in Sentences 25 and 26-3. “A good superintendent” is explained by the expanded clause following the colon, just as “articles” in No. 25 is explained by the details that follow the colon.
In most sentences like No. 64 the second clause can be tied to the first by a conjunction (for, as, etc.); but the exact idea is not thus expressed. While this would sometimes show the sense relation between the clauses, it would distinctly weaken the force and beauty of the language, tending to make the second clause a specific, rather than a general, statement.
The differentiation between the semicolon and the colon in Nos. 63 and 64 is very clear if we recognize the colon as the mark of apposition or explanation. In No. 63 other clauses are added to the first to add other thoughts; in No. 64 the added clause repeats the thought of the first by way of amplification of such thought.
The greatest value of a mark of punctuation lies in its ability to indicate a meaning that depends upon the relation of one group of words to another; and so long as language has a literary value, just so long will the colon be useful as an aid in the expression of thought. This is illustrated in our next sentence (No. 65), where a semicolon, instead of the colon, would suggest the and relation, and thus make havoc of the sense: