“It’s not kept in a bottle, but in ampoules which hold the exact amount and are then thrown away. There aren’t any kicking about in the theatre. I’ll beat some up for you to see if you like.”

“Very good of you, Mr. Thoms. I’m being a crashing bore, I’m afraid.”

Thoms protested his freedom from boredom and fussed away. Alleyn prowled meditatively round the theatre until the fat man returned.

“Here we are,” said Thoms cheerfully. “Here are ampoules of oil and camphor. Here’s the anti-gas serum and here’s the hyoscine solution. All labelled, as you see. Tell you what I’ll do. I’ll set out the table as it would have been for the op. How will that do you?”

“Splendid!”

“Let’s see now—ampoules here, serum there. Here’s the bottle of hyoscine solution; thought you’d want to see that too. Old-fashioned idea—it should be in ampoules, but matron’s a bit of a dug-out.”

“The bottle’s nearly full, I see.”

“Yes. I believe one injection had been given.”

Alleyn noted mentally that this tallied with Nurse Harden’s and the scally’s impression that the bottle had been full before the operation and had since been used once.

“Can anyone have access to this bottle?” asked Alleyn suddenly.

“What? Oh, yes—any of the theatre staff.”

“May I have a small amount—I may have to get it tested?”

He produced a tiny bottle from his pocket and Thoms, looking rather intrigued, filled it with the solution.

“There you are. Now—where were we? Oh! Along here, small syringe for the camphor, another small syringe for the hyoscine—they hold twenty-five minims each. That would be the one Sir John would use for his tablet. Now the whopper for the serum. It holds ten c.c.’s.”

“Ten c.c.’s?”

“That’s about a hundred and sixty minims,” explained Thoms.

“What’s that in gallons?”

Thoms looked at the inspector as if he had uttered something in Chinese and then burst out laughing.

“Not quite as solid at that,” he said. “One hundred and sixty minims is equal to two and two-thirds drachms. That any better?”

“Not much,” grumbled Alleyn. “The dawn may break later on. I’m talking like Nurse Banks. What’s the strength of this hyoscine?”

“Quarter per cent.”

“But—what does that mean? They’ll have to get someone cleverer than me for this game.”

“Cheer up. It’s one grain in one point one ounces of water.”

“That sounds as though it means something. I must look up those horrid little things at the end of an arithmetic-book. Wait a moment, now. Don’t say a word, Mr. Thoms, if you please,” begged Alleyn. “I’m doing sums.”