be avoided by raising one of the sides of the
jar, A, for a moment, and slipping in the little cup, D, with the
combustible body as quickly as possible. In this manner of operating, a
small quantity of common air gets into the jar, but it is so very
inconsiderable as not to injure either the progress or accuracy of the
experiment in any sensible degree.

When the cup, D, is introduced under the jar, we suck out a part of the
oxygen gas, so as to raise the mercury to EF, as formerly directed, Part
I. Chap. V. otherwise, when the combustible body is set on fire, the gas
becoming dilated would be in part forced out, and we should no longer be
able to make any accurate calculation of the quantities before and after
the experiment. A very convenient mode of drawing out the air is by
means of an air-pump syringe adapted to the syphon, GHI, by which the
mercury may be raised to any degree under twenty-eight inches. Very
inflammable bodies, as phosphorus, are set on fire by means of the
crooked iron wire, MN, Pl. IV. Fig. 16. made red hot, and passed quickly
through the mercury. Such as are less easily set on fire have a small
portion of tinder, upon which a minute particle of phosphorus is fixed,
laid upon them before using the red hot iron.

In the first moment of combustion the air, being heated, rarifies, and
the mercury descends; but when, as in combustions of phosphorus and
iron, no elastic fluid is formed, absorption becomes presently very
sensible, and the mercury rises high into the jar. Great attention must
be used not to burn too large a quantity of any substance in a given
quantity of gas, otherwise, towards the end of the experiment, the cup
would approach so near the top of the jar as to endanger breaking it by
the great heat produced, and the sudden refrigeration from the cold
mercury. For the methods of measuring the volume of the gasses, and for
correcting the measures according to the heighth of the barometer and
thermometer, &c. see Chap. II. Sect. V. and VI. of this part.

The above process answers very well for burning all the concrete
substances, and even for the fixed oils: These last are burnt in lamps
under the jar, and are readily set on fire by means of tinder,
phosphorus, and hot iron. But it is dangerous for substances susceptible
of evaporating in a moderate heat, such as ether, alkohol, and the
essential oils; these substances dissolve in considerable quantity in
oxygen gas; and, when set on fir