results into court; had kept no laboratory notes; relied
solely on a memory so deficient that although he had been teaching for
thirty-five years, he could not tell the shape of a crystal of tartar
emetic, the poison in question; and upon the stand made a statement
different from one which he had furnished officially to the district
attorney of Baltimore fourteen months before.

There are principles of toxicology which ought to have legal force
and recognition, and ought to govern expert testimony in the same
way that the principles of evidence govern ordinary testimony.
Without presuming to enumerate these, I will cite two or three for
illustration. Certain substances, the so-called irritant poisons, such
as arsenic, tartar emetic and the like, induce their toxic effects
by causing irritation and inflammation of the alimentary canal. All
authorities agree that poisoning by these substances cannot be proved,
or even rendered, very probable, by symptoms alone--that chemical
evidence, the discovery of the poison in the food, dejections, or in
case of death the body, is absolutely essential for making out a case.
Irritation and inflammation of the alimentary canal occur so often
and so suddenly from natural causes, which are sometimes apparent, but
often hidden, that no especial weight can be attached to them.

In the case of the so-called neurotic poisons, those which act upon
the nervous system, the symptoms are so closely simulated by natural
disease that even when they agree in the most absolute manner with
those usually developed by any such poison they only render poisoning
highly probable, not certain.[15] When in any case the symptoms
diverge from the typical array, poisoning becomes improbable just in
proportion to the amount of divergence.

All toxicological authorities also agree that in the case of the
metallic poisons, such as tartar emetic and arsenic, the metal must be
brought into court, and that the so-called "color tests" are not to be
relied on. When sulphuretted hydrogen is passed through solutions of
these metallic substances colored precipitates are thrown down, which
at one time were thought to be absolute proof of the existence of the
poison in the original solution. But in the celebrated Donnal case,
tried at Falmouth, England, in 1817, Dr. Neale saved the accused by
showing that a decoction of onions, of which the deceased had eaten a
short time before death, yielded similar precipitates to tho