The next investigation of this plant was made by Maisch in 1864. He
criticizes Khittel's experiments as follows: "It is well known that the
_exhalations_ of _Rhus toxicodendron_ exert a poisonous influence on the
human body; the poisonous principle must, therefore, be volatile and, at
the same time, be naturally in such a loose state of combination as to
be continually eliminated and separated with the usual products of
vegetable exhalations. It is natural to suppose that during the process
of drying, the greatest portion of the poisonous principle should be
lost. The loss must be still greater if the dried leaves are powdered, a
hot infusion prepared from them, and this infusion evaporated down to
the original weight of the dried leaves. It is obvious that Dr. Khittel
could not have selected a better method for obtaining the least possible
quantity of the poisonous principle, if, indeed, it could be obtained by
this process at all."
Maisch then worked up 8-3/4 ounces of the leaves of the plant in a way
to get the alkaloid, making some improvements on Khittel's method, but
failed to find it. Believing that the poison was a volatile acid, he
enclosed some fresh leaves of the plant in a tin box with several test
papers. The blue litmus paper became red showing the presence of an
acid. He concluded from this experiment that the exhalations of the
leaves contained a volatile organic acid which he thought was the
poisonous substance. To determine this point, he prepared the acid in
larger quantity by macerating the leaves with water, expressing and
distilling the expressed juice. He was poisoned in doing this work
although he had not been affected by handling the living plant and had
considered himself immune. He obtained an acid which investigation
showed to be somewhat like formic acid, more like acetic acid, but
having some reactions different from both. "Taking all the reactions
together, it is unquestionably a new organic acid for which I propose
the name of _Toxicodendric Acid_," writes Maisch. He further says: "That
it is the principle to which poison oak owes its effects on the human
system was proved to my entire satisfaction by the copious eruption and
formation of numerous vesicles on the back of my hand, on the fingers,
wrists, and bare arms while I was distilling and operating with it.
Several persons coming into the room while I was engaged with it were
more or less poisoned by the vapours diffused in the room; and I even
transferred the poisonous effects to some persons, merely by shaking
hands with them.
"The diluted acid, as obtained by me, and stronger solutions of its
salts, were applied to several persons, and eruptions were produced in
several instances, probably by the former, though not always, which was
most likely owing to the dilute state of the acid. Whenever this was
boiled, I always felt the same itching sensation in the face, and on the
bare arms, which I experience on continual exposure of my hands to the
juice of the plant."
Toxicodendric acid was thought to be the active principle from the time
of Maisch's work until the investigation by Pfaff in 1895.