Words and phrases such as these, used as exclamations, are not true
interjections, for they express a little more than feeling. They express
an idea which, in our haste, we do not completely express. The other
words necessary to the expression of the idea are omitted because of the
stress of emotion. For example:
Silence! I will hear no more.
In this sentence it is understood that we mean, _Let us have silence, I
will hear no more_. But in the stress of our emotion, we have omitted
the words, _Let us have_.
If we say, _Good! that will do splendidly_, you know that we mean, _That
is good_, we have simply omitted _That is_, which is necessary to
complete the sentence. Sometimes when we are greatly excited we abandon
our sentence construction altogether and use only the most important
words. For example:
A sail! a sail!
This is not a sentence, for it does not contain a verb, yet we know that
what was meant was, _I see a sail, I see a sail_.