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 its lower
extremity t immersed in the liquor. If any absorption takes place,
either in the retort, or in any of the bottles, a sufficient quantity of
external air enters, by means of these tubes, to fill up the void; and
we get rid of the inconvenience at the price of having a small mixture
of common air with the products of the experiment, which is thereby
prevented from failing altogether. Though these tubes admit the external
air, they cannot permit any of the gasseous substances to escape, as
they are always shut below by the water of the bottles.
It is evident that, in the course of experiments with this apparatus,
the liquor of the bottles must rise in these tubes in proportion to the
pressure sustained by the gas or air contained in the bottles; and this
pressure is determined by the height and gravity of the column of fluid
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