“That is pretty conclusive,” observed Thorndyke, lifting the stopper out of the reagent bottle, “but we will apply the final test.”  He dropped a few drops of the hypochlorite solution on to the tile, and immediately the black stain faded away and vanishod.  “We can now answer your question, Mr. Barton,” said he, replacing the stopper as he turned to our client.  “The specimen that you brought us certainly contains arsenic, and in very considerable quantities.”

“Then,” exclaimed Mr. Barton, starting from his chair, “you will come and help me to rescue my brother from this dreadful peril.  Don’t refuse me, Dr. Thorndyke, for mercy’s sake, don’t refuse.”

Thorndyke reflected for a moment.

“Before we decide,” said he, “we must see what engagements we have.”

With a quick, significant glance at me, he walked into the office, whither I followed in some bewilderment, for I knew that we had no engagements for the evening.

“Now, Jervis,” said Thorndyke, as he closed the office door, “what are we to do?”

“We must go, I suppose,” I replied.  “It seems a pretty urgent case.”

“It does,” he agreed.  “Of course, the man may be telling the truth, after all.”

“You don’t think he is, then?”

“No.  It is a plausible tale, but there is too much arsenic in that arrowroot.  Still, I think I ought to go.  It is an ordinary professional risk.  But there is no reason why you should put your head into the noose.”

“Thank you,” said I, somewhat huffily.  “I don’t see what risk there is, but if any exists I claim the right to share it.”

“Very well,” he answered with a smile, “we will both go.  I think we can take care of ourselves.”

He re-entered the sitting-room, and announced his decision to Mr. Barton, whose relief and gratitude were quite pathetic.

“But,” said Thorndyke, “you have not yet told us where your brother lives.”

“Rexford,” was the reply—­“Rexford, in Essex.  It is an out-of-the-way place, but if we catch the seven-fifteen train from Liverpool Street, we shall be there in an hour and a half.”

“And as to the return?  You know the trains, I suppose?”

“Oh yes,” replied our client; “I will see that you don’t miss your train back.”

“Then I will be with you in a minute,” said Thorndyke; and, taking the still-bubbling flask, he retired to the laboratory, whence he returned in a few minutes carrying his hat and overcoat.

The cab which had brought our client was still waiting, and we were soon rattling through the streets towards the station, where we arrived in time to furnish ourselves with dinner-baskets and select our compartment at leisure.

During the early part of the journey our companion was in excellent spirits.  He despatched the cold fowl from the basket and quaffed the rather indifferent claret with as much relish as if he had not had a single relation in the world, and after dinner he became genial to the verge of hilarity.  But, as time went on, there crept into his manner a certain anxious restlessness.  He became silent and preoccupied, and several times furtively consulted his watch.