He waited until the front door had closed behind her, and then, having lit himself a cigarette, climbed back into the car, and started off again in the direction of Campden Hill.
It was a few minutes after midnight when he let himself in at the outer gate of the Red Lodge. The light was still burning in the hall, and, knowing from experience that the Professor frequently continued his work until the early hours of the morning, he walked straight through to the laboratory and tapped lightly on the door.
As he half expected, there was a creak of footsteps inside, and the Professor himself, wearing an old Jaeger dressing gown, appeared on the threshold. At the same moment a peculiarly acrid and unpleasant smell drifted past him into the passage.
"So you've come back, eh?" he said in his queer, high-pitched voice. "I hope you enjoyed your evening?"
"I didn't do badly," replied Colin. "I went to Shadwell, saw my friends, had a forty-mile run in the car, and finished up with a dinner and a theatre."
The old man nodded grimly. "It sounds rather an exhausting form of recreation. I should imagine that after all that you must be quite ready for your bed."
"Oh, I'm not tired," said Colin, "not in the slightest. If you've got anything on hand I should like to come in and make myself useful."
"You'll do nothing of the sort," returned the Professor. "You will help yourself to a whisky and soda, and then you will go straight upstairs to your room."
Colin hesitated. "And how about you, sir? Surely you've done enough work for to-day?"
"I shall be following you shortly," was the answer. "I am only waiting to see the result of a small experiment."