blood in a little distilled water.

Before him was his new spectroscope, and I could see that now he
was satisfied with what the uncannily delicate light-detective had
told him. He pricked his finger and let a drop of blood fall into
a little fresh distilled water, some of which he placed in the
spectroscope.

"Look through it," he said. "Blood diluted with water shows the
well-known dark bands between D and E, known as the oxyhemoglobin
absorption." I looked as he indicated and saw the dark bands.
"Now," he went on, "I add some of this other liquid."

He picked up a bottle of something with a faint greenish tinge.

"See the bands gradually fade?"

I watched, and indeed they did diminish in intensity and finally
disappear, leaving an uninterrupted and brilliant spectrum.

"My spectroscope," he said, simply, "shows that the blood-crystals
of Barnes are colorless. Barnes was poisoned--by some gas, I
think. I wish I had time to hunt along the road where the accident
took place." As he said it, he walked over and drew from a cabinet
several peculiar arrangements made of gauze.

He was about to say something more when there came a knock at the
door. Kennedy shoved the gauze arrangements into his pocket and
opened it. It was Maude Euston, breathless and agitated.