He pushed the bottle across to Thorndyke, who carried it to the window, and, extracting a small quantity of the contents with a glass rod, examined the pasty mass with the aid of a lens; then, lifting the bell-glass cover from the microscope, which stood on its table by the window, he smeared a small quantity of the suspected matter on to a glass slip, and placed it on the stage of the instrument.
"I observe a number of crystalline particles in this," he said, after a brief inspection, "which have the appearance of arsenious acid."
"Ah!" ejaculated Mr. Barton, "just what I feared. But are you certain?"
"No," replied Thorndyke; "but the matter is easily tested."
He pressed the button of the bell that communicated with the laboratory, a summons that brought the laboratory assistant from his lair with characteristic promptitude.
"Will you please prepare a Marsh's apparatus, Polton," said Thorndyke.
"I have a couple ready, sir," replied Polton.
"Then pour the acid into one and bring it to me, with a tile."
As his familiar vanished silently, Thorndyke turned to Mr. Barton.
"Supposing we find arsenic in this arrowroot, as we probably shall, what do you want us to do?"