If two or more clauses grammatically complete and not joined by a conjunction are to form a single compound sentence, the proper mark of punctuation is a semicolon.
Mary Shelley’s works are entertaining; they are full of engaging ideas.
It is nearly half past five; we cannot reach town before dark.
It is, of course, equally correct to write each of these as two sentences, replacing the semicolons with periods.
Mary Shelley’s works are entertaining. They are full of engaging ideas.
It is nearly half past five. We cannot reach town before dark.
If a conjunction is inserted, the proper mark is a comma. (Rule 4.)
Mary Shelley’s works are entertaining, for they are full of engaging ideas.
It is nearly half past five, and we cannot reach town before dark.
A comparison of the three forms given above will show clearly the advantage of the first. It is, at least in the examples given, better than the second form because it suggests the close relationship between the two statements in a way that the second does not attempt, and better than the third because it is briefer and therefore more forcible. Indeed, this simple method of indicating relationship between statements is one of the most useful devices of composition. The relationship, as above, is commonly one of cause and consequence.
Note that if the second clause is preceded by an adverb, such as accordingly, besides, then, therefore, or thus, and not by a conjunction, the semicolon is still required.
I had never been in the place before; besides, it was dark as a tomb.
An exception to the semicolon rule is worth noting here. A comma is preferable when the clauses are very short and alike in form, or when the tone of the sentence is easy and conversational.
Man proposes, God disposes.
The gates swung apart, the bridge fell, the portcullis was drawn up.
I hardly knew him, he was so changed.
Here today, gone tomorrow.