arc current, and the arc reproduces sounds and speech distinctly, loud enough to be heard several feet.  Indeed, the arc could be used as a transmitter, too, if a sensitive receiver replaced the transmitter at the other end.  The things needed are an arc-lamp, an impedance coil, or small transformer-coil, a rheostat, and a source of energy.  The alternating current is not adapted to reproduce speech, but the ordinary direct current is.  Of course, the theory isn’t half as simple as the apparatus I have described.”

He had unscrewed the Osram lamp.  The talking ceased immediately.

“Two investigators named Ort and Ridger have used a lamp like this as a receiver,” he continued.  “They found that words spoken were reproduced in the lamp.  The telephonic current variations superposed on the current passing through the lamp produce corresponding variations of heat in the filament, which are radiated to the glass of the bulb, causing it to expand and contract proportionately, and thus transmitting vibrations to the exterior air.  Of course, in sixteen-and thirty-two-candle-power lamps the glass is too thick, and the heat variations are too feeble.”

Who was it whose voice Brixton had recognised as familiar over Kennedy’s hastily installed detectaphone?  Certainly he must have been a scientist of no mean attainment.  That did not surprise me, for I realised that from that part of Europe where this mystical Red Brotherhood operated some of the most famous scientists of the world had sprung.

A hasty excursion into the basement netted us nothing.  The place was deserted.

We could only wait.  With parting instructions to Brixton in the use of the detectaphone we said good night, were met by a watchman and escorted as far as the lodge safely.

Only one remark did Kennedy make as we settled ourselves for the long ride in the accommodation train to the city.  “That warning means that we have two people to protect—­both Brixton and his daughter.”

Speculate as I might, I could find no answer to the mystery, nor to the question, which was also unsolved, as to the queer malady of Brixton himself, which his physician diagnosed as jaundice.

VI

THE DETECTAPHONE

Far after midnight though it had been when we had at last turned in at our apartment, Kennedy was up even earlier than usual in the morning.  I found him engrossed in work at the laboratory.

“Just in time to see whether I’m right in my guess about the illness of Brixton,” he remarked, scarcely looking up at me.

He had taken a flask with a rubber stopper.  Through one hole in it was fitted a long funnel; through another ran a glass tube, connecting with a large U-shaped drying-tube filled with calcium chloride, which in turn connected with a long open tube with an up-turned end.

Into the flask Craig dropped some pure granulated zinc coated with platinum.  Then he covered it with dilute sulphuric acid through the funnel tube.  “That forms hydrogen gas,” he explained, “which passes through the drying-tube and the ignition-tube.  Wait a moment until all the air is expelled from the tubes.”