"Hail, holy light! offspring of heaven just born!"—Milton.
"But thou, O Hope! with eyes so fair,
What was thy delighted measure?"—Collins.
It will be observed that the interjection O is an exception to the rule: it is often followed by a comma, but never by an exclamation-point.
An exclamation-point sometimes gives the same words quite another meaning. The difference between "What's that?" and "What's that!" is obvious.
The Dash.—Cobbett did not favor the use of this mark, as we see from the following: "Let me caution you against the use of what, by some, is called the dash. The dash is a stroke along the line; thus, 'I am rich—I was poor—I shall be poor again.' This is wild work indeed! Who is to know what is intended by these dashes? Those who have thought proper, like Mr. Lindley Murray, to place the dash amongst the grammatical points, ought to give us some rule relative to its different longitudinal dimensions in different cases. The inch, the three-quarter-inch, the half-inch, the quarter-inch: these would be something determinate; but 'the dash,' without measure, must be a perilous thing for the young grammarian to handle. In short, 'the dash' is a cover for ignorance as to the use of points, and it can answer no other purpose."
This is one of the few instances in which Cobbett was wrong. The dash is the proper point with which to mark an unexpected or emphatic pause, or a sudden break or transition. It is very often preceded by another point. "And Huitzilopochtli—a sweet name to roll under one's tongue—for how many years has this venerable war-god blinked in the noonday sun!" "Crowds gathered about the newspaper bulletins, recalling the feverish scenes that occurred when the President's life was thought to be hanging by a thread. 'Wouldn't it be too bad,' said one, 'if, after all—no, I won't allow myself to think of it.'" "Was there ever—but I scorn to boast." "You are—no, I'll not tell you what you are."
"He suffered—but his pangs are o'er;
Enjoyed—but his delights are fled;
Had friends—his friends are now no more;
And foes—his foes are dead."—Montgomery.
"Greece, Carthage, Rome,—where are they?" "He chastens;—but he chastens to save."
Dashes are much used where parentheses were formerly employed. "In the days of Tweed the expression to divide fair—forcible, if not grammatical—acquired much currency." "In truth, the character of the great chief was depicted two thousand five hundred years before his birth, and depicted—such is the power of genius—in colors which will be fresh as many years after his death." "To render the Constitution perpetual—which God grant it may be!—it is necessary that its benefits should be practically felt by all parts of the country."
Parenthesis.—This mark is comparatively little used nowadays. The dash is preferred, probably because it disfigures the page less. The office of the parenthesis is to isolate a phrase which is merely incidental, and which might be omitted without detriment to the grammatical construction.