"How long will your experiment take, Alan ?" he said in a calm indifferent voice.

The presence of a third person in the room seemed to give him extraordinary courage.
Campbell frowned and bit his lip.

"It will take about five hours," he answered.
"It will be time enough, then, if you are back at half-past seven, Francis.

Or stay: just leave my things out for dressing.

You can have the evening to yourself.