An interesting piece of engineering work has recently been
accomplished at Bristol, England, which consisted in the moving of a
foot-bridge 134 feet in length, bodily, down the river a considerable
distance. The pontoons by means of which the bridge was floated to its
new position consisted of four 80-ton barges, braced together so as to
form one solid structure 64 feet in width, and were placed in position
soon after the tide commenced to rise. At six o'clock A.M. the top of
the stages, which was 24 feet above the water, touched the under part
of the bridge, and in a quarter of an hour later both ends rose from
their foundations. When the tide had risen 4 ft. the stage and bridge
were floated to the new position, when at 8.30 the girders dropped on
to their beds.
* * * * *
THE GENERATION OF STEAM, AND THE THERMODYNAMIC PROBLEMS
INVOLVED.[1]
[Footnote 1: Lecture delivered at the Institution of Civil
Engineers, session 1883-84. For the illustrations we are indebted
to the courtesy of Mr. J. Forrest, the secretary.]
By Mr. WILLIAM ANDERSON, M.I.C.E.
It will not be necessary to commence this lecture by explaining the
origin of fuel; it will be sufficient if I remind you that it is to
the action of the complex rays of the sun upon the foliage of plants
that we mainly owe our supply of combustibles. The tree trunks and
branches of our forests, as well as the subterranean deposits of coal
and naphtha, at one time formed portions of the atmosphere in the form
of carbonic acid gas; that gas was decomposed by the energy of the
solar rays, the carbon and the oxygen were placed in positions of
advantage with respect to each other--endowed with potential energy;
and it is my duty this evening to show how we can best make use of
these relations, and by once more combining the constituents of fuel
with the oxygen of the air, reverse the action which caused the growth
of the plants, that is to say, by destroying the plant reproduce the
heat and light which fostered it. The energy which can be set free by
this process cannot be greater than that derived originally from the
sun, and which, acting through the frail mechanism of green leaves,
tore asunder the strong bonds of chemical affinity wherein the carbon
and oxygen were hound, converting the former into the ligneous
portions of the plants and setting the latter free for other uses. The
power thus silently exerted is enormous; for every ton of carbon
separated in twelve hours necessitates an expenditure of energy
represented by at least 1,058 horse power, but the action is spread
over an enormous area of leaf surface, rendered necessary by the small
proportion of carbonic acid contained in the air, by measure only
1/2000 part, and hence the action is silent and imperceptible. It is
now conceded on all hands that what is termed heat is the energy of
molecular motion, and that this motion is convertible into various
kinds and obeys the general laws relating to motion. Two substances
brought within the range of chemical affinity unite with more or less
violence; the motion of transition of the particles is transformed,
wholly or in part, into a vibratory or rotary motion, either of the
particles themselves or the interatomic ether; and according to the
quality of the motions we are as a rule, besides other effects, made
conscious of heat or light, or of both. When these emanations come to
be examined they are found to be complex in the extreme, intimately
bound up together, and yet capable of being separated and analyzed.
As soon as the law of definite chemical combination was firmly
established, the circumstance that changes of temperature accompanied
most chemical combinations was noticed, and chemists were not long in
suspecting that the amount of heat developed or absorbed by chemical
reaction should be as much a property of the substances entering into
combination as their atomic weights. Solid ground for this expectation
lies in the dynamic theory of heat. A body of water at a given height
is competent by its fall to produce a definite and invariable quantity
of heat or work, and in the same way two substances falling together
in chemical union acquire a definite amount of kinetic energy, which,
if not expended in the work of molecular changes, may also by suitable
arrangements be made to manifest a definite and invariable quantity of
heat.
At the end of last century Lavoisier and Laplace, and after them, down
to our own time, Dulong, Desprez, Favre and Silbermann, Andrews,
Berthelot, Thomson, and others, devoted much time and labor to the
experimental determination of the heat of combustion and the laws
which governed its development. Messrs. Favre and Silbermann, in
particular, between the years 1845 and 1852, carried out a splendid
series of experiments by means of the apparatus partly represented in
Fig. 1 (opposite), which is a drawing one-third the natural size of
the calorimeter employed. It consisted essentially of a combustion
chamber formed of thin copper, gilt internally. The upper part of the
chamber was fitted with a cover through which the combustible could be
introduced, with a pipe for a gas jet, with a peep hole closed by
adiathermanous but transparent substances, alum and glass, and with a
branch leading to a thin copper coil surrounding the lower part of the
chamber and descending below it. The whole of this portion of the
apparatus was plunged into a thin copper vessel, silvered internally
and filled with water, which was kept thoroughly mixed by means of
agitators. This second vessel stood inside a third one, the sides and
bottom of which were covered with the skins of swans with the down on,
and the whole was immersed in a fourth vessel tilled with water, kept
at the average temperature of the laboratory. Suitable thermometers of
great delicacy were provided, and all manner of precautions were taken
to prevent loss of heat.
[Illustration: THE GENERATION OF STEAM. Fig 1.]
It is impossible not to admire the ingenuity and skill exhibited in
the details of the apparatus, in the various accessories for
generating and storing the gases used, and for absorbing and weighing
the products of combustion; but it is a matter of regret that the
experiments should have been carried out on so small a scale. For
example, the little cage which held the solid fuel tested was only 5/8
inch diameter by barely 2 inches high, and held only 38 grains of
charcoal, the combustion occupying about sixteen minutes. Favre and
Silbermann adopted the plan of ascertaining the weight of the
substances consumed by calculation from the weight of the products of
combustion. Carbonic acid was absorbed by caustic potash, as also was
carbonic oxide, after having been oxidized to carbonic acid by heated
oxide of copper, and the vapor of water was absorbed by concentrated
sulphuric acid. The adoption of this system showed that it was in any
case necessary to analyze the products of combustion in order to
detect imperfect action. Thus, in the case of substances containing
carbon, carbonic oxide was always present to a variable extent with
the carbonic acid, and corrections were necessary in order to
determine the total heat due to the complete combination of the
substance with oxygen. Another advantage gained was that the
absorption of the products of combustion prevents any sensible
alteration in the volumes during the process, so that corrections for
the heat absorbed in the work of displacing the atmosphere were not
required. The experiments on various substances were repeated many
times. The mean results for those in which we are immediately
interested are given in Table I., next column.
Comparison with later determinations have established their
substantial accuracy. The general conclusion arrived at is thus
stated:
"As a rule there is an equality between the heat disengaged or
absorbed in the acts, respectively, of chemical combination or
decomposition of the same elements, so that the heat evolved during
the combination of two simple or com-pound substances is equal to the
heat absorbed at the time of their chemical segregation."