at
do you want us to do?"

"I want you to come and see my brother," replied our client.

"Why not take a note from me to his doctor?"

"No, no; I want you to come--I should like you both to come--and put a
stop at once to this dreadful business. Consider! It's a matter of life
and death. You won't refuse! I beg you not to refuse me your help in
these terrible circumstances."

"Well," said Thorndyke, as his assistant reappeared, "let us first see
what the test has to tell us."

Polton advanced to the table, on which he deposited a small flask, the
contents of which were in a state of brisk effervescence, a bottle
labelled "calcium hypochlorite," and a white porcelain tile. The flask
was fitted with a safety-funnel and a glass tube drawn out to a fine
jet, to which Polton cautiously applied a lighted match. Instantly there
sprang from the jet a tiny, pale violet flame. Thorndyke now took the
tile, and held it in the flame for a few seconds, when the appearance of
the surface remained unchanged save for a small circle of condensed
moisture. His next proceeding was to thin the arrowroot with distilled
water until it was quite fluid, and then pour a small quantity into the
funnel. It ran slowly down the tube into the flask, with the bubbling
contents of which it became speedily mixed. Almost immediately a change
began to appear in the character of the flame, which from a pale violet
turned gradually to a sickly blue, while above it hung a faint cloud of
white smoke. Once more Thorndyke held the tile above the jet, but this
time, no sooner had the pallid flame touched the cold surface of the
porcelain, than there appeared on the latter a glistening black stain.

"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."

Wi