Let us note how carefully language is used in this sentence: the letter possesses a singular charm (it never loses its novelty) and a perpetual charm (it never grows old).
The colon both groups the language and shows the relation (apposition) between the two main groups. On the other hand, the semicolon in this sentence stands where the sense relation is the and relation.
It is not quite proper to say that the second and third clauses are explanatory simply of “charm”: they are explanatory of the entire first clause, repeating and expanding the thought expressed in that clause, just as what follows the colon in the sentence we are writing explains what precedes.
Our next illustrative sentence is from one of Howell’s novels, which was printed at The Riverside Press. It has the fault of No. 26, and the added fault of an indefinite but relation:
27. He was not candid; he did not shun concealments and evasions; but positive lies he had kept from.
The second clause is clearly explanatory of the first; and the third clause simply modifies the second by showing the degree of untruthfulness of the man.
With the new meaning of the colon we are now considering, a colon after “candid” informs the reader of the relation between the main groups of the sentence:
27-1. He was not candid: he did not shun concealments and evasions; but positive lies he had kept from.
So far as the mere grouping is concerned, this could be done in Nos. 26 and 27 by a semicolon and a comma in each; but the semicolon would not inform the reader of the true relation between the two larger groups. The colon would still be required if the next two groups took a comma, or even no mark, between them.
We called this relation that of apposition. We might say, somewhat more specifically, that the second group is an amplification in language to express an extension of the idea, or to fortify the image, of the first group. This purpose may also be accomplished by a contrast between the ideas expressed in the two groups.