filter immersed in the mass, and with the aid of a suction-pump, the
dark brown solution is drawn off; then fresh ether is poured on to the
lupuline, and it is allowed to stand for another twenty-four hours.
After this process has been three times repeated, nearly everything the
petroleum will dissolve has probably been extracted. The solutions are
put together, and the petroleum ether distilled off _in vacuo_ at a low
temperature, until there remains in the flask a dark brown sirup, which
on cooling solidifies into a crystalline mass. This is pulverized and
turned on to a filter composed of a large funnel, in which a smaller
funnel covered with muslin is inserted. With the aid of a suction-pump,
the greater portion of the thick, crude solution can be filtered
through. There remains on the filter a highly colored crystalline
"cake," which should be pulverized with a small quantity of petroleum
ether and again filtered. After this operation has been repeated three
or four times, we obtain an almost colorless mass, consisting of
hop-bitter acid, contaminated by small quantities of a fatty substance,
and a substance which I could not isolate, and which I had at first
great trouble in separating from the hop-bitter acid.

If we do not wish to utilize this crude substance at once, it will be
necessary to melt it in the water bath and pour it into a bottle under
close seal, where it will at once crystallize and solidify. If it
remains exposed to the atmosphere, it will soon become sticky and
turn partly into resin. Six kilos of lupuline, which included a large
proportion of sand, furnished 400 grammes of crude hop-bitter acid. The
first experiments in crystallization with petroleum ether gave poor
results; it is difficult to produce the acid pure in large quantities
by this process, as a small quantity of the above substance obstinately
clings to it, and it readily assumes a non-crystallizable form. Our
object is more readily attained if we crystallize it once from alcohol,