I recall that as I followed him, I half imagined that the floor gave way just a bit, and there flashed over me, by a queer association of ideas, the recollection of having visited an amusement park not long before where merely stepping on an innocent-looking section of the flooring had resulted in a tremendous knocking and banging beneath, much to the delight of the lovers of slap-stick humor.  This was serious business, however, and I quickly banished the frivolous thought from my mind.

“The discovery of poison, and its identification,” began Craig at last when we had all arrived and were seated about him, “often involves not only the use of chemistry but also a knowledge of the chemical effect of the poison on the body, and the gross as well as microscopic changes which it produces in various tissues and organs—­changes, some due to mere contact, others to the actual chemicophysiological reaction between the poison and the body.”

His hand was resting on the poles of a large battery, as he proceeded:  “Every day the medical detective plays a more and more important part in the detection of crime, and I might say that, except in the case of crime complicated by a lunacy plea, his work has earned the respect of the courts and of detectives, while in the case of insanity the discredit is the fault rather of the law itself.  The ways in which the doctor can be of use in untangling the facts in many forms of crime have become so numerous that the profession of medical detective may almost be called a specialty.”

Kennedy repeated what he had already told me about electrolysis, then placed between the poles of the battery a large piece of raw beef.

He covered the negative electrode with blotting paper and soaked it in a beaker near at hand.

“This solution,” he explained, “is composed of potassium iodide.  In this other beaker I have a mixture of ordinary starch.”

He soaked the positive electrode in the starch and then jammed the two against the soft red meat.  Then he applied the current.

A few moments later he withdrew the positive electrode.  Both it and the meat under it were blue!

“What has happened?” he asked.  “The iodine ions have actually passed through the beef to the positive pole and the paper on the electrode.  Here we have starch iodide.”

It was a startling idea, this of the introduction of a substance by electrolysis.

“I may say,” he resumed, “that the medical view of electricity is changing, due in large measure to the genius of the Frenchman, Dr. Leduc.  The body, we know, is composed largely of water, with salts of soda and potash.  It is an excellent electrolyte.  Yet most doctors regard the introduction of substances by the electric current as insignificant or nonexistent.  But on the contrary the introduction of drugs by electrolysis is regular and far from being insignificant may very easily bring about death.