e gas driven over into B by
raising the bottle F. The gas is drawn back into A by lowering F and
when the solution in B has reached the mark in the capillary tube, the
cock I is closed and a reading is taken on the burette, the level of the
water in the bottle F being brought to the same level as the water in A.
The operation is repeated until a constant reading is obtained, the
number of cubic centimeters being the percentage of CO_{2} in the flue
gases.

The gas is then driven over into the pipette C and a similar operation
is carried out. The difference between the resulting reading and the
first reading gives the percentage of oxygen in the flue gases.

The next operation is to drive the gas into the pipette D, the gas being
given a final wash in E, and then passed into the pipette C to
neutralize any hydrochloric acid fumes which may have been given off by
the cuprous chloride solution, which, especially if it be old, may give
off such fumes, thus increasing the volume of the gases and making the
reading on the burette less than the true amount.

The process must be carried out in the order named, as the pyrogallol
solution will also absorb carbon dioxide, while the cuprous chloride
solution will also absorb oxygen.

As the pressure of the gases in the flue is less than the atmospheric
pressure, they will not of themselves flow through the pipe connecting
the flue to the apparatus. The gas may be drawn into the pipe in the way
already described for filling the apparatus, but this is a tedious
method. For rapid work a rubber bulb aspirator connected to the air
outlet of the cock G will enable a new supply of gas to be drawn into
the pipe, the apparatus then being filled as already described. Another
form of aspirator draws the gas from the flue in a constant stream, thus
insuring a fresh supply for each sample.

The analysis made by the Orsat apparatus is volumetric; if the analysis
by weight is required, it can be found from the volumetric analysis as
follows:

Multiply the percentages by volume by either the densities or the
molecular weight of each gas, and divide the products by the sum of all
the products; the quotients will be the percentages by weight. For most
work sufficient accuracy is secured by using the even values of the
molecular weights.

The even values of the molecular weights of the gases appearing in an
analysis by an Orsat are:

Carbon Dioxide 44
Carbon Monoxide 28
Oxygen