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PREFACE TO THE FRENCH EDITION                 xiii
combustible gas. In order that the latter may burn it must be
mixed intimately with the oxygen; this requires a certain period
of time, and the mixture must be made at a temperature sufficiently
high to permit the gases to react upon each other.
In steam-boiler settings, an arch of refractory materials is
frequently built over the fire. But if this arch does not form a
pocket in which the hot gases may accumulate and remain for a
time, the unburned gases will flow too rapidly from beneath the
arch, and the effect of the arch will be only imaginary. The work
of Professor Groume-Grjimailo is filled with similar examples,
with complete information regarding the works in which the
observations were made. This work is not, therefore, purely
theoretical; it is a thoroughly practical treatise based upon actual
observations and experiments.
Not content with having developed these new ideas, the
author has endeavored to place them before his readers in a man-
ner which will make them absolutely clear, just as he has been
doing for a long time in the instruction of his classes. In order to
present in a visible manner the circulation of the hot gases within
the furnace, he has employed small models of furnace sections,
enclosed between two plates of glass, and within these models he
has arranged to circulate .two liquids of different densities, water
serving as the heavier liquid and colored kerosene as the lighter
liquid, representing the hot gases or flame. The localization of
the current of kerosene shows very clearly whether the furnaces
are of poor design or not. This method is particularly applicable
for presenting this subject before those who may not be well
informed regarding the technical principles of the great industries;
but it has the inconvenience of giving, to a certain extent, an
erroneous view of the actual phenomena. During its circulation,
the colored kerosene cannot be changed into water or mixed with
it, while the light or hottest gases, having given up their heat, are
transformed into colder and heavier gases. This introduces very
essential differences which must be clearly understood. The
only purpose of this method of representation is to present to the
eye a very strong impression which will stimulate the imagination;
in practical application it is necessary that we consider these
phenomena as they really occur and study the gas itself, which
can only be conceived in the abstract and of which no visible
representation can be made.