Semicolons have two principal uses:
1. Use a semicolon to join two independent clauses into a single sentence.
Senator Petrie is out kissing babies again. This must be an election year.
Senator Petrie is out kissing babies again; this must be an election year.
2. Use a semicolon to separate items in a series when those items contain internal punctuation.
I've lived in Hattiesburg, Mississippi; Detroit, Michigan; Manhattan, Kansas; and Boise, Idaho.
Semicolons help the reader to see the individual items more clearly than would be possible with commas alone.
4.19 Re-write the following sentences using semicolons whenever appropriate.
a. By now, Marie Antoinette had totally lost favor with the people. They would believe nothing she said.
b. In the midst of this confusion, Arnold did his best to keep calm however, he was unsuccessful.
c. When you purchase a camera, you will want to consider its features, both convenience and performance related, its lens quality, probably the most important single factor, and its price.
d. The GNP is not a very sensitive economic indicator, still it tells us the essentials.
e. The place was empty, the lights were out.