[Footnote: An address before the Paris Academy.]
By M. DUMAS.
Of all the gases that the atmosphere contains, there is one which offers
a special interest, as well on account of the part ascribed to it in
the mutual interchange going on between the two organic kingdoms, as
on account of the relation that it has been observed to occupy between
earth, air, and water; this gas is carbonic acid.
Ever since the fact has been established that animals consume oxygen
and give out carbonic acid as the product of respiration, while plants
consume carbonic acid and give out oxygen, the question has often been
asked whether the quantity of carbonic acid contained in the air did not
represent a sort of sustaining reservoir which was being continually
drawn on by the plants and resupplied by animals, so that it has
doubtless remained unchanged owing to this double action.
On the other hand, Boussingault has long since shown that volcanic
regions give out through crevices and fumaroles enormous quantities of
carbonic acid. The deposition of carbonate of lime that is continually
taking place on the sea-bottom is, on the other hand, fixing carbonic
acid in quantities which we may accurately estimate from the strata of
limestone seen on the surface of the earth. We might imagine, that in
comparison with the huge volumes of carbonic acid sent forth in volcanic
districts, even in the oldest one, and the mass of carbonate of lime
deposited on the sea bottom, the results attributed to the life of
plants and animals would be of no consequence either for increasing or
diminishing the physiological carbonic acid in the air comparable with
those which are accomplished by the purely geological exchange.
Schloesing has recently succeeded, by a happy application of the
principle of dissociation, in showing that the amount of carbonic acid
in the air bears a direct relation to the quantity of bicarbonate
of lime dissolved in sea water. If the quantity of carbonic acid
diminishes, the bicarbonate of the water is decomposed, half of its
carbonic acid escapes into the atmosphere, and the neutral carbonate of
lime is precipitated. The aqueous vapor condensed from the air dissolves
part of the carbonic acid contained therein, and carries it along, when
it falls as rain upon the earth, and takes up there enough lime to form
the bicarbonate, which is thus carried back to the sea.
The physiological role of carbonic acid, its geognostic influence, and
its relations to most ordinary meteorological phenomena on the earth's
surface--all these contribute to give special weight to studies
concerned in the estimation of the normal quantity of carbonic acid in
the air.
Nevertheless, this estimation is attended with great difficulty. Not
everyone is able to take up such questions, and not all processes are
adapted to it. The first thought which would naturally arise would be to
inclose a known volume of air in a given vessel, and then determine its
carbonic acid by measuring or weighing it. In this way we should obtain
the exact relation between a volume of air and the volume of carbonic
acid in it, for any given moment, and in any given place. If, however,
this be done with a ten-liter flask, for example, it would only hold
3 c.c. of carbonic acid, weighing 6 milligrammes; and, whether it is
weighed or measured, the error may easily equal 10 per cent. of the real
value, hence no deductions could be drawn from the observed facts.
For this reason larger volumes of air were taken, and a current of air,
whose volume could be accurately measured by known methods, was passed
through condensers capable of retaining the carbonic acid. But in this
case the air must pass very slowly through it, so that the process may
last several hours; and since the air is continually in motion, owing to
vertical and horizontal currents, the experiment may be begun with the
air of one place, and concluded with air from a far distant spot. For
example, if an experiment lasting twenty-four hours was made in Paris
when the air moved but four meters per second (nine or ten miles per
hour), it might be begun with air from the Department of the Seine, and
end with air from the Department of the Rhone, or the Belgian frontier,
according to the direction of the wind.
So long as we had no analytical methods of sufficient delicacy to
estimate with certainty the hundredth, or at least the tenth of a
milligramme of carbonic acid, it was very difficult to determine the
quantity in the air at a given time and place. It is frequently possible
to analyze upon the plain air that has descended from the heights
above, and to examine by bright daylight the effect of night upon the
atmosphere.
Still other difficulties show themselves in such investigations. It
seems very easy to collect carbonic acid in potash tubes, and to
determine its amount from the increase in weight of the tubes; but,
alas! to how many sources of error is this method exposed. If the potash
has been in contact with any organic substance, it will absorb oxygen.
If the pumice that takes the place of the potash contains protoxide of
iron, it will also absorb oxygen. In both cases the oxygen increases the
weight of the carbonic acid.
Every experimenter who has been compelled to repeat the weighing of a
somewhat complicate piece of apparatus, with an interval of several
hours between, knows how many inaccuracies he is exposed to if he is
compelled to take into calculation the changes of temperature and
pressure, and the moisture on the surface of the apparatus. After
fighting all these difficulties, and frequently in vain, the
experimenter begins to mistrust every result that depends only on
difference in weight, and to prefer those methods whereby the substance
to be estimated can be isolated, so that it can be seen and handled,
weighed or measured, in a free state, and in its own natural condition.
The classical experiments of Thenard, of Th. de Saussure, of Messrs.
Boussingault, on the quantity of carbonic acid in the air, are well
known to every one: they need only to be organized, repeated, and
multiplied.
J. Reiset, who has conducted a long and tedious series of experiments on
this subject, has adopted a process that seems to offer every guarantee
of accuracy. The air that furnishes the carbonic acid is aspirated
through the absorption apparatus by two aspirators of 600 liters
capacity. The temperature and pressure of the air are carefully
measured. The carbonic acid is absorbed by baryta water in three bulb
apparatus. The last bulb, which serves as a check to control the
operation, remains clear, and proves that no binoxide of barium
is formed. The baryta water used is titrated before and after the
operation, and from the difference is calculated the quantity of
carbonate formed, and hence of the carbonic acid.
These tedious experiments, which varied in duration from 6 to 25 hours,
require at least two days of continuous labor. They were repeated
193 times by Reiset in 1872, 1873, and 1879. They were made in still
weather, and in violent winds and storms. The air was taken at the
sea-shore, in the middle of the fields, on the level earth, during
harvests, in the forests, and in Paris. Under such varied conditions,
the quantity of carbonic acid varied but little; the numbers obtained
were between 2.94 and 3.1, which may be taken as a general average of
the carbonic acid in the air.
The quantity of carbonic acid in the free atmosphere is tolerably
constant, which must necessarily be the case according to Schloesing's
proposed relation between the bi-carbonate of lime in the sea and the
carbonic acid in the air. The only cause that seems at all competent to
change the geological quantity of carbonic acid in the atmosphere is
the formation of fog. As the aqueous vapors condense, they collect the
carbonic acid; and the foggy air, as a rule, is more heavily laden with
this gas than ordinary air.
It is not surprising that there is less carbonic acid in the air
collected on clear summer days, in the midst of clover, etc., that is in
an active reducing furnace; if anything is surprising, it is that the
quantity of carbonic acid does not sink below 2.8.
It is also a matter for surprise that in Paris, among so many sources of
carbonic acid, the furnace fires, the respiration of men and animals,
and the spontaneous decomposition and decay of organic substances, the
quantity of carbonic acid does not exceed 3.5.
If, then, the great general mean of normal atmospheric carbonic acid
deviates but little from 2.9 or 3.0, it is not doubtful that under local
conditions, in closed places, and under exceptional meteorological
conditions, considerable variations may occur in these proportions. But
these variations do not affect the general laws of the composition of
the atmosphere.
There are two entirely distinct points from which the measurement of the
atmospheric carbonic acid may be contemplated.
The first consists in considering it as a geological element which
belongs to the gaseous envelope of the earth in general, and it leads us
to express the general relation of carbonic acid to the quantity of air,
as about three volumes in 10,000.
The second, which relates to accidental and local phenomena, to the
activity of man and beast, to the effect of fires and of decomposing
organic matter, to volcanic emanations, and finally to the action of
clouds and rain, permits us to recognize the changes which can occur
in air exposed to the influences mentioned, and to a certain extent
confined. Without denying that it is of interest from a meteorological
and hygienic standpoint, it does not take the same rank as first.
J. Reiset's experiments, by their number, accuracy, the large volumes
employed, and the interval of years that separate them, have definitely
established two facts on which the earth's history must depend: the
first is, that the percentage of carbonic acid in the air scarcely
changes; the second, that it differs but little from three
ten-thousandths by volume.
These results are fully confirmed by the results which were obtained by
Franz Schulze, in Rostock, in 1868, 1869, 1870, and 1871. The averages
which he got, with very small variation, were 2.8668 for 1869, 2.9052
for 1870, and 3.0126 for 1871.
More recently Muentz and Aubin have analyzed air collected on the plains
near Paris, on the Pic du Midi, and on the top of Puy-de-Dome. Their
results agree with those published by Reiset and Schulze.
The grand average of carbonic oxide in the air seems to be tolerably
fixed, but after this starting-point is established it remains to study
the variations that it is capable of, not from local causes, which are
of little importance, but from general causes connected with large
movements of the air. Upon this study, which demands the co-operation of
a definite number of observers stationed at different and distant
points of the earth, the experiments being made simultaneously and by
comparable methods.
M. Dumas called the attention of the Academy to this point, in
connection with its mission of selecting suitable stations for observing
the transit of Venus. The process and apparatus of Muentz and Aubin
offer the means adapted for making these experiments, and seem
sufficient to solve the problem which science proposes, of determining
the present quantity of carbonic acid in the air.
If these experiments yield satisfactory results, as we have good reasons
to believe they will, it is to be hoped that annual observations will be
made in properly-chosen places, so as to determine the variations which
may possibly take place in the relative quantity of atmospheric carbonic
acid during the coming century.--_Compt. Rend_., p. 589.
[Although this proposition was made by a Frenchman to his fellow
scientists, would it not be well for some American to accept the
challenge, and bring it before the coming meeting of the American
Association for the Advancement of Science, in the hope that we,
too, may contribute our mite of effort in the same direction?--_Ed.
Knowledge_.]
* * * * *
THE INFLUENCE OF AQUEOUS VAPOR ON THE EXPLOSION OF CARBONIC OXIDE AND
OXYGEN.
[Footnote: Read before the British Association, Southampton Meeting,
Section B, 1882.]
By HAROLD B. DIXON, M.A., Millard Lecturer in Chemistry, Balliol and
Trinity Colleges, Oxford.
Two years ago I had the honor of showing before the Chemical Section of
the British Association some experiments, in which a well-dried mixture
of carbonic oxide and oxygen was submitted to electric sparks without
exploding.[1] It was further shown that the introduction of a very
minute quantity of aqueous vapor into the non-explosive mixture was
sufficient to cause explosive combination between the gases when the
spark was passed. The hypothesis advanced to account for the observed
facts was that carbonic oxide does not unite directly with oxygen at
a high temperature, but only indirectly through the intervention of
water-vapor present, a molecule of water being decomposed by one of
carbonic oxide to form a molecule of carbonic acid and one of free
hydrogen, and the latter uniting with the oxygen to re-form a molecule
of water, which again undergoes the same cycle of changes, till all the
oxygen is transferred to the carbonic oxide: