salt is 5,5 + 14,4 + 2 = 21,9
The free acid 2,75 + ,6 = 3,35
Gas 42,5
Water 32,25
——————
100
Now about 78,1 grains of salt were decompounded, and formed into 42,5
grains of gas, 3,35 grains acid, and 32,25 grains water.
But there is every reason to suppose, that in this process, when the
hydrogene of the ammoniac combines with a portion of the oxygene of
the nitric acid to form water, and the nitrogene enters into union
with the nitrogene and remaining oxygene of the nitric acid, to form
nitrous oxide; that water pre-existing in nitric acid and ammoniac,
such as they existed in the aëriform state, is deposited with the water
produced by the new arrangement, and not wholly combined with the
nitrous oxide formed. Hence it is impossible to determine with great
exactness, the quantity of water which was absolutely formed in this
experiment.
78,1 grains of salt are composed of 15,4 alkali, 58 acid, and 4,7 water.
And reasoning from the different affinities of water for nitric acid,
ammoniac, and nitrous oxide, it is probable that ammoniac, in its
decomposition, divides its water in such a ratio, between the nitrogene
furnished to the nitrous oxide, and the hydrogene entering into union
with the oxygene of the nitric acid, as to enable us to assume, that
the hydrogene requires for its saturation nearly the same quantity of
oxygene as when in the aëriform state; or that it certainly cannot
require less.
But 15,4 alkali contain 3,08 hydrogene, and 12,32 nitrogene;[87] and
3,08 hydrogene require 17,4 of oxygene to form 20,48 of water.
[87] Owing part of their weight to an unknown quantity of water.
Now 32,5 grains of water existed before the experiment; 4,7 grains of
water were contained by the salt decomposed, and 32,5-4,7 = 27,8: and
27,8-20,48, the quantity generated, = 7,52, the quantity existing in
the nitric acid.
But the nitric acid decomposed is 58ᵍ-3,35 = to 54,7; and 54,7-7,5
= 47,2, which entered into new combinations. These 47,2 consist of
33,2 oxygene, and 14, nitrogene. And 33,2-17,4, the quantity employed
to form the water, = 15,8, which combined with 14,0, nitrogene of
the nitric acid, and 12,32 of that of the ammoniac, to form 42,12 of
nitrous oxide. And on this estimation, 100 parts of nitrous oxide
would contain 37,6 oxygene, and 62,4 nitrogene; a computation much
nearer the results of the analysis than could have been expected,
particularly as so many unavoidable sources of error existed in the
process.
The experiment that I have detailed is the most accurate of four, made
on the same quantity of salt. The others were carried on at rather
higher temperatures, in consequence of which, more water and salt were
sublimed with the gas.
To Berthollet, we owe the discovery of the products evolved during the
slow decomposition of nitrate of ammoniac; but as this philosopher
in his examination of this process, chiefly designed to prove the
existence of hydrogene in ammoniac, he did not ascertain the quantity
of gas produced, or minutely examine its properties; from two of
them, its absorption by water and its capability of supporting the
vivid combustion of a taper, he inferred its identity with the
dephlogisticated nitrous gas of Priestley, and concluded that it was
nitrous gas with excess of pure air.[88]
[88] Mem. de Paris. 1785, and Journal de Physique, 1786, page 175.
VII. _Of the heat produced during the decomposition of nitrate of
ammoniac._
To ascertain whether the temperature of nitrate of ammoniac was
increased or diminished after it had been raised to the point essential
to its decomposition, during the evolution of nitrous oxide and water;
that is, in common language, whether heat was generated or absorbed
in the process; I introduced a thermometer into about 1500 grains of
fibrous nitrate of ammoniac, rendered liquid in a deep porcelain cup.
During the whole of the evaporation, the temperature was about 380°,
the fire being carefully regulated.
As soon as the decomposition took place, the thermometer began to rise;
in less than a quarter of a minute it was 410°, in two minutes it was
460°.
The cup was removed from the fire; the decomposition still went on
rapidly, and for about a minute the thermometer was stationary. It
then gradually and slowly fell; in three minutes it was 440°, in
five minutes 420°, in seven minutes 405° in nine minutes 360° and in
thirteen minutes 307°, when the decomposition had nearly ceased, and
the salt began to solidify.
From this experiment, it is evident that an increase of temperature
is produced by the decomposition of nitrate of ammoniac: though the
capacity of water and nitrous oxide for heat, supposing the truth of
the common doctrine, and reasoning from analogy, must be considerably
greater than that of the salt.
VIII. _Of the decomposition of Nitrate of Ammoniac at high
temperatures, and production of Nitrous gas, Nitrogene, Nitrous Acid,
and Water._
At an early period of my investigation relating to the nitrous oxide,
I discovered that when a heat above 600° was applied to nitrate of
ammoniac, so that a vivid luminous appearance was produced in the
retort, certain portions of nitrous gas, and nitrogene, were evolved
with the nitrous oxide. But I was for some time ignorant of the
precise nature of this decomposition, and doubtful with regard to
the possibility of effecting it in such a manner as to prevent the
production of nitrous oxide altogether.
I first attempted to decompose nitrate of ammoniac at high
temperatures, by introducing it into a well coated green glass retort,
having a wide neck, communicating with the pneumatic apparatus, and
strongly heated in an air-furnace. But though in this process a
detonation always took place, and much light was produced, yet still
the greater portion of the gas generated was nitrous oxide; the nitrous
gas and nitrogene never amounting to more than one third of the whole.
After breaking many retorts by explosions, without gaining any accurate
results, I employed a porcelain tube, curved so as to be capable of
introduction into the pneumatic apparatus, and closed at one end.
The closed end was heated red, nitrate of ammoniac introduced into it,
and all the latter portions of gas produced in the explosion, received
in the pneumatic apparatus, filled with warm water.
Three explosions were required to fill a jar of the capacity of 20
cubic inches. The gas produced in the first, when it came over, was
transparent and dark orange, similar in its appearance to the nitrous
acid gas produced in the first experiment; but it speedily became white
and clouded, whilst a slight diminution of volume took place.
When the second portion was generated and mingled with the clouded
gas, it again became transparent and yellow for a short time, and then
assumed the same appearance as before.
The water in the trough, after this experiment, had an acid taste, and
quickly reddened cabbage juice rendered green by an alkali.
6 cubic inches of the gas produced were exposed to boiled water, but
little or no absorption took place. Hence, evidently, it contained no
nitrous oxide.
They were then exposed to solution of sulphate of iron: the solution
quickly became dark colored, and an absorption of 1,6 took place on
agitation.[89]
[89] The absorption of nitrous gas by sulphate of iron, &c. will be
treated of in the next division.
The gas remaining instantly extinguished the taper, and was
consequently nitrogene.
This experiment was repeated, with nearly the same results.
We may then conclude, that at high temperatures, nitrate of ammoniac is
wholly resolved into water, nitrous acid, nitrous gas, and nitrogene;
whilst a vivid luminous appearance is produced.
The transparency and orange color produced in the gas that had been
clouded, by new portions of it, doubtless arose from the solution of
the nitric acid and water forming the cloud, in the heated nitrous
vapor produced, so as to constitute an aëriform triple compound;
whilst the cloudiness and absorption subsequent were produced by the
diminished temperature, which destroyed the ternary combination, and
separated the nitrous acid and water from the nitrous gas.
From the rapidity with which the deflagration of nitrate of ammoniac
proceeds, and from the immense quantity of light produced, it is
reasonable to suppose that a very great increase of temperature takes
place. The tube in which the decomposition has been effected, is always
ignited after the process.