deal,” said Dick Manton. “If it is chemical, I should be disposed to include Barakoff; he knows more about chemistry than all the others put together. But in any case, there is as yet nothing we can even begin to work on.”

A fortnight went past. The death-roll in England had assumed terrible proportions, and apparently the authorities were as far off as ever from coming to grips with the mystery. But a clue came through the heroism of a London policeman.

One night Constable Jervis was patrolling a beat which led him through some tumbledown streets in the lowest quarter of Canning Town. Suddenly he caught sight of a man rushing from a small empty house. At once Jervis started in pursuit of the man, who was running hard away from him. As he did so, there came the sound of an explosion, and the house the man had just left collapsed like a pack of cards. At the same time the odour of the dreaded violet vapour completely filled the narrow street.

The Terror had attacked London, and Jervis knew that to cross that zone of vapour meant certain death.

He did not hesitate. Muffling his face with his pocket handkerchief as he ran, he dashed at full speed after the stranger, whom he could just discern. He crossed the zone of death, almost overpowered by the curious scent of petrol and musk that loaded the still air, and a moment later was in pursuit, blowing his whistle loudly as he ran. A moment later a second policeman, hearing his colleague’s whistle, stood at the end of the road barring the way. The desperado was trapped.

Snatching out a revolver, the man backed against the wall and opened fire on his pursuers who were rapidly closing in on him. But both the policemen were armed, and both opened fire. Jervis’s second shot killed the man on the spot.

He proved to be a well-known member of a Russian anarchist group which had its head-quarters in the slums of Soho. The gallant Jervis had faced certain death—as a matter of fact he was among the hundred or so victims when the epidemic broke out twelve days later—but he had done his duty in accordance with the splendid traditions of the force to which he belonged.

The source of the mysterious epidemic was now, to a certain extent, localised. It needed no great acumen to guess the motive and origin of the fiendish plot. But to discover the master-mind which held the full solution of the mystery was another matter.

The first step was a 

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