By far the most valuable work on _Rhus toxicodendron_ is that of Pfaff.
From a clinical study of Rhus poisoning, Pfaff came to the conclusion
that the poison must be a non-volatile skin irritant. The more volatile
the irritant, the quicker is its action on the skin. Formic acid acts
very quickly; acetic acid, less volatile than formic, acts more slowly,
but still much more quickly than poison ivy, the latent period of which
is usually from two to five days. Pfaff thought that the volatile acid
obtained by Maisch might have contained some of the poisonous principle
as an impurity, but that it would not produce the dermatitis if prepared
in a pure state. He therefore prepared a quantity of the acid by
distilling the finely divided fresh plant with steam. The yield was
increased by acidulating the mixture with sulphuric acid before the
distillation. The acid distillate so obtained was freed from a
non-poisonous oily substance by shaking the solution with ether. Barium
and sodium salts were made by neutralizing the acid, and were purified
by crystallization. Analysis showed them to be salts of acetic acid, and
they gave the characteristic tests for this acid. The toxicodendric acid
of Maisch was thus shown to be acetic acid, and was therefore not the
poisonous principle of the plant.
Pfaff obtained the active principle by the following process: The plant
was extracted with alcohol, the alcohol was distilled off, and the
residue was taken up in ether. The ether solution was washed with water
and dilute sodium carbonate solution, and the ether was evaporated. An
oily, black, poisonous substance partly soluble in alcohol was obtained.
To get the active principle in a pure state, this residue was extracted
with alcohol and filtered and the filtrate was precipitated fractionally
by lead acetate. The final precipitates consisted of the lead compound
of the poison in a pure state. On decomposing the lead compounds with
ammonium sulphide, shaking out with ether, and letting the ether
evaporate spontaneously, a non-volatile oil was obtained which gave the
characteristic skin eruptions. The pure lead compounds made in different
preparations were analyzed and assigned the formula C_{21}H_{30}O_{4}Pb.
The oil itself was not analyzed. Pfaff proposed the name _Toxicodendrol_
for the oil. He found that it was not volatile, was decomposed by heat,
was soluble in alcohol, ether, chloroform, benzene, etc., but insoluble
in water. Its effects upon the human skin were studied in many
experiments upon himself and others. It was shown that an exceedingly
minute quantity of the poison will produce the dermatitis, even 1/1000
milligram applied in olive oil being active. The oil was given
internally to rabbits, its effects being most marked on the kidneys.
The oil obtained by Pfaff from _Rhus venenata_ seemed to be identical
with that from _Rhus toxicodendron_.