s. The more volatile substances, as the
lighter oils, ammoniac, and several others, will condense in the
recipient GC, whilst the gasses, which are not susceptible of
condensation by cold, will pass on by the tubes, and boil up through the
liquors in the several bottles. Such as are absorbable by water will
remain in the first bottle, and those which caustic alkali can absorb
will remain in the others; whilst such gasses as are not susceptible of
absorption, either by water or alkalies, will escape by the tube RM, at
the end of which they may be received into jars in a pneumato-chemical
apparatus. The charcoal and fixed earth, &c. which form the substance or
residuum, anciently called _caput mortuum_, remain behind in the retort.

In this manner of operating, we have always a very material proof of the
accuracy of the analysis, as the whole weights of the products taken
together, after the process is finished, must be exactly equal to the
weight of the original substance submitted to distillation. Hence, for
instance, if we have operated upon eight ounces of starch or gum arabic,
the weight of the charry residuum in the retort, together with that of
all the products gathered in its neck and the balloon, and of all the
gas received into the jars by the tube RM added to the additional weight
acquired by the bottles, must, when taken together, be exactly eight
ounces. If the product be less or more, it proceeds from error, and the
experiment must be repeated until a satisfactory result be procured,
which ought not to differ more than six or eight grains in the pound
from the weight of the substance submitted to experiment.

In experiments of this kind, I for a long time met with an almost
insurmountable difficulty, which must at last have obliged me to desist
altogether, but for a very simple method of avoiding it, pointed out to
me by Mr Hassenfratz. The smallest diminution in the heat of the
furnace, and many other circumstances inseparable from this kind of
experiments, cause frequent reabsorptions of gas; the water in the
cistern of the pneumato-chemical apparatus rushes into the last bottle
through the tube RM, the same circumstance happens from one bottle into
another, and the fluid is often forced even into the recipient C. This
accident is prevented by using bottles having three necks, as
represented in the plate, into one of which, in each bottle, a capillary
glass-tube St, st, st, st, is adapted, so as to have it