[1] See Victor Loret. "Les Flutes egyptiennes antiques," _Journal
Asiatique_, 8eme serie, tome xiv., Paris, 1889, pp. 129, 130 and 132.
[2] _Catalogue descriptif et analytique du musee du Conservatoire
Royal de Bruxelles_ (Ghent, 1880), p. 141.
[3] _A Descriptive Catalogue of the Musical Instruments in the South
Kensington Museum_, by Carl Engel (London, 1874), p. 143.
ARGOL, the commercial name of crude tartar (q.v.). It is a
semi-crystalline deposit which forms on wine vats, and is generally grey
or red in colour.
ARGON (from the Gr. [Greek: a-], privative, and [Greek: ergon], work;
hence meaning "inert"), a gaseous constituent of atmospheric air. For
more than a hundred years before 1894 it had been supposed that the
composition of the atmosphere was thoroughly known. Beyond variable
quantities of moisture and traces of carbonic acid, hydrogen, ammonia,
&c., the only constituents recognized were nitrogen and oxygen. The
analysis of air was conducted by determining the amount of oxygen
present and assuming the remainder to be nitrogen. Since the time of
Henry Cavendish no one seemed even to have asked the question whether
the residue was, in truth, all capable of conversion into nitric acid.
The manner in which this condition of complacent ignorance came to be
disturbed is instructive. Observations undertaken mainly in the interest
of Prout's law, and extending over many years, had been conducted to
determine afresh the densities of the principal gases--hydrogen, oxygen
and nitrogen. In the latter case, the first preparations were according
to the convenient method devised by Vernon Harcourt, in which air
charged with ammonia is passed over red-hot copper. Under the influence
of the heat the atmospheric oxygen, unites with the hydrogen of the
ammonia, and when the excess of the latter is removed with sulphuric
acid, the gas properly desiccated should be pure nitrogen, derived in
part from the ammonia, but principally from the air. A few concordant
determinations of density having been effected, the question was at
first regarded as disposed of, until the thought occurred that it might
be desirable to try also the more usual method of preparation in which
the oxygen is removed by actual oxidation of copper without the aid of
ammonia. Determinations made thus were equally concordant among
themselves, but the resulting density was about 1/1000 part greater than
that found by Harcourt's method (Rayleigh, _Nature_, vol. xlvi. p. 512,
1892). Subsequently when _oxygen_ was substituted for air in the first
method, so that all (instead of about one-seventh part) of the nitrogen
was derived from ammonia, the difference rose to 1/2%. Further
experiment only brought out more clearly the diversity of the gases
hitherto assumed to be identical. Whatever were the means employed to
rid air of accompanying oxygen, a uniform value of the density was
arrived at, and this value was 1/2% greater than that appertaining to
nitrogen extracted from compounds such as nitrous oxide, ammonia and
ammonium nitrite. No impurity, consisting of any known substance, could
be discovered capable of explaining an excessive weight in the one case,
or a deficiency in the other. Storage for eight months did not disturb
the density of the chemically extracted gas, nor had the silent electric
discharge any influence upon either quality. ("On an Anomaly encountered
in determining the Density of Nitrogen Gas," _Proc. Roy. Soc._, April
1894.)
At this stage it became clear that the complication depended upon some
hitherto unknown body, and probability inclined to the existence of a
gas in the atmosphere heavier than nitrogen, and remaining unacted upon
during the removal of the oxygen --a conclusion afterwards fully
established by Lord Rayleigh and Sir William Ramsay. The question which
now pressed was as to the character of the evidence for the universally
accepted view that the so-called nitrogen of the atmosphere was all of
one kind, that the nitrogen of the air was the same as the nitrogen of
nitre. Reference to Cavendish showed that he had already raised this
question in the most distinct manner, and indeed, to a certain extent,
resolved it. In his memoir of 1785 he writes:--
"As far as the experiments hitherto published extend, we scarcely know
more of the phlogisticated part of our atmosphere than that it is not
diminished by lime-water, caustic alkalies, or nitrous air; that it is
unfit to support fire or maintain life in animals; and that its
specific gravity is not much less than that of common air; so that,
though the nitrous acid, by being united to phlogiston, is converted
into air possessed of these properties, and consequently, though it
was reasonable to suppose, that part at least of the phlogisticated
air of the atmosphere consists of this acid united to phlogiston, yet
it may fairly be doubted whether the whole is of this kind, or whether
there are not in reality many different substances confounded together
by us under the name of phlogisticated air. I therefore made an
experiment to determine whether the whole of a given portion of the
phlogisticated air of the atmosphere could be reduced to nitrous acid,
or whether there was not a part of a different nature to the rest
which would refuse to undergo that change. The foregoing experiments
indeed, in some measure, decided this point, as much the greatest part
of air let up into the tube lost its elasticity; yet, as some remained
unabsorbed, it did not appear for certain whether that was of the same
nature as the rest or not. For this purpose I diminished a similar
mixture of dephlogisticated [oxygen] and common air, in the same
manner as before [by sparks over alkali], till it was reduced to a
small part of its original bulk. I then, in order to decompound as
much as I could of the phlogisticated air [nitrogen] which remained in
the tube, added some dephlogisticated air to it and continued the
spark until no further diminution took place. Having by these means
condensed as much as I could of the phlogisticated air, I let up some
solution of liver of sulphur to absorb the dephlogisticated air; after
which only a small bubble of air remained unabsorbed, which certainly
was not more than 1/120 of the bulk of the dephlogisticated air let up
into the tube; so that, if there be any part of the dephlogisticated
air of our atmosphere which differs from the rest, and cannot be
reduced to nitrous acid, we may safely conclude that it is not more
than 1/120 part of the whole."
Although, as was natural, Cavendish was satisfied with his result, and
does not decide whether the small residue was genuine, it is probable
that his residue was really of a different kind from the main bulk of
the "phlogisticated air," and contained the gas afterwards named argon.
[Illustration: FIG. 1.]
The announcement to the British Association in 1894 by Rayleigh and
Ramsay of a new gas in the atmosphere was received with a good deal of
scepticism. Some doubted the discovery of a new gas altogether, while
others denied that it was present in the atmosphere. Yet there was
nothing inconsistent with any previously ascertained fact in the
asserted presence of 1% of a non-oxidizable gas about half as heavy
again as nitrogen. The nearest approach to a difficulty lay in the
behaviour of liquid air, from which it was supposed, as the event proved
erroneously, that such a constituent would separate itself in the solid
form. The evidence of the existence of a new gas (named Argon on account
of its chemical inertness), and a statement of many of its properties,
were communicated to the Royal Society (see _Phil. Trans._ clxxxvi. p.
187) by the discoverers in January 1895. The isolation of the new
substance by removal of nitrogen from air was effected by two distinct
methods. Of these the first is merely a development of that of
Cavendish. The gases were contained in a test-tube A (fig. 1) standing
over a large quantity of weak alkali B, and the current was conveyed in
wires insulated by U-shaped glass tubes CC passing through the liquid
and round the mouth of the test-tube. The inner platinum ends DD of the
wire may be sealed into the glass insulating tubes, but reliance should
not be placed upon these sealings. In order to secure tightness in spite
of cracks, mercury was placed in the bends. With a battery of five Grove
cells and a Ruhmkorff coil of medium size, a somewhat short spark, or
arc, of about 5 mm. was found to be more favourable than a longer one.
When the mixed gases were in the right proportion, the rate of
absorption was about 30 c.c. per hour, about thirty times as fast as
Cavendish could work with the electrical machine of his day. Where it is
available, an alternating electric current is much superior to a battery
and break. This combination, introduced by W. Spottiswoode, allows the
absorption in the apparatus of fig. 1 to be raised to about 80 c.c. per
hour, and the method is very convenient for the purification of small
quantities of argon and for determinations of the amount present in
various samples of gas, e.g. in the gases expelled from solution in
water. A convenient adjunct to this apparatus is a small voltameter,
with the aid of which oxygen or hydrogen can be introduced at pleasure.
The gradual elimination of the nitrogen is tested at a moment's notice
with a miniature spectroscope. For this purpose a small Leyden jar is
connected as usual to the secondary terminals, and if necessary the
force of the discharge is moderated by the insertion of resistance in
the primary circuit. When with a fairly wide slit the yellow line is no
longer visible, the residual nitrogen may be considered to have fallen
below 2 or 3%. During this stage the oxygen should be in considerable
excess. When the yellow line of nitrogen has disappeared, and no further
contraction seems to be in progress, the oxygen maybe removed by
cautious introduction of hydrogen. The spectrum may now be further
examined with a more powerful instrument. The most conspicuous group in
the argon spectrum at atmospheric pressure is that first recorded by A.
Schuster (fig. 2). Water vapour and excess of oxygen in moderation do
not interfere seriously with its visibility. It is of interest to note
that the argon spectrum may be fully developed by operating upon a
miniature scale, starting with only 5 c.c. of air (_Phil. Mag._ vol. i.
p. 103, 1901).
The development of Cavendish's method upon a large scale involves
arrangements different from what would at first be expected. The
transformer working from a public supply should give about 6000 volts on
open circuit, although when the electric flame is established the
voltage on the platinums is only from 1600 to 2000. No sufficient
advantage is attained by raising the pressure of the gases above
atmosphere, but a capacious vessel is necessary. This may consist of a
glass sphere of 50 litres' capacity, into the neck of which, presented
downwards, the necessary tubes are fitted. The whole of the interior
surface is washed with a fountain of alkali, kept in circulation by
means of a small centrifugal pump. In this apparatus, and with about one
horse-power utilized at the transformer, the absorption of gas is 21
litres per hour ("The Oxidation of Nitrogen Gas," _Trans. Chem. Soc._,
1897).
In one experiment, specially undertaken for the sake of measurement, the
total air employed was 9250 c.c., and the oxygen consumed, manipulated
with the aid of partially de-aerated water, amounted to 10,820 c.c. The
oxygen contained in the air would be 1942 c.c.; so that the quantities
of atmospheric nitrogen and of total oxygen which enter into combination
would be 7308 c.c. and 12,762 c.c. respectively. This corresponds to N +
1.75 O, the oxygen being decidedly in excess of the proportion required
to form nitrous acid. The argon ultimately found was 75.0 c.c., or a
little more than 1% of the atmospheric nitrogen used. A subsequent
determination over mercury by A.M. Kellas (_Proc. Roy. Soc._ lix. p.
66, 1895) gave 1.186 c.c. as the amount of argon present in 100 c.c. of
mixed atmospheric nitrogen and argon. In the earlier stages of the
inquiry, when it was important to meet the doubts which had been
expressed as to the presence of the new gas in the atmosphere, blank
experiments were executed in which air was replaced by nitrogen from
ammonium nitrite. The residual argon, derived doubtless from the water
used to manipulate the gases, was but a small fraction of what would
have been obtained from a corresponding quantity of air.
[Illustration: FIG. 2.]
The other method by which nitrogen may be absorbed on a considerable
scale is by the aid of magnesium. The metal in the form of thin turnings
is charged into hard glass or iron tubes heated to a full red in a
combustion furnace. Into this air, previously deprived of oxygen by
red-hot copper and thoroughly dried, is led in a continuous stream. At
this temperature the nitrogen combines with the magnesium, and thus the
argon is concentrated. A still more potent absorption is afforded by
calcium prepared _in situ_ by heating a mixture of magnesium dust with
thoroughly dehydrated quick-lime. The density of argon, prepared and
purified by magnesium, was found by Sir William Ramsay to be 19.941 on
the O = 16 scale. The volume actually weighed was 163 c.c. Subsequently
large-scale operations with the same apparatus as had been used for the
principal gases gave an almost identical result (19.940) for argon
prepared with oxygen.
Argon is soluble in water at 12 deg. C. to about 4.0%, that is, it is
about 2-1/2 times more soluble than nitrogen. We should thus expect to
find it in increased proportion in the dissolved gases of rain-water.
Experiment has confirmed this anticipation. The weight of a mixture of
argon and nitrogen prepared from the dissolved gases showed an excess of
24 mg. over the weight of true nitrogen, the corresponding excess for
the atmospheric mixture being only 11 mg. Argon is contained in the
gases liberated by many thermal springs, but not in special quantity.
The gas collected from the King's Spring at Bath gave only 1/2%, i.e.
half the atmospheric proportion.
The most remarkable physical property of argon relates to the constant
known as the ratio of specific heats. When a gas is warmed one degree,
the heat which must be supplied depends upon whether the operation is
conducted at a constant volume or at a constant pressure, being greater
in the latter case. The ratio of specific heats of the principal gases
is 1.4, which, according to the kinetic theory, is an indication that an
important fraction of the energy absorbed is devoted to rotation or
vibration. If, as for Boscovitch points, the whole energy is
translatory, the ratio of specific heats must be 1.67. This is precisely
the number found from the velocity of sound in argon as determined by
Kundt's method, and it leaves no room for any sensible energy of
rotatory or vibrational motion. The same value had previously been found
for mercury vapour by Kundt and Warburg, and had been regarded as
confirmatory of the monatomic character attributed on chemical grounds
to the mercury molecule. It may be added that helium has the same
character as argon in respect of specific heats (Ramsay, _Proc. Roy.
Soc._ l. p. 86, 1895).
The refractivity of argon is .961 of that of air. This low refractivity
is noteworthy as strongly antagonistic to the view at one time favoured
by eminent chemists that argon was a condensed form of nitrogen
represented by N3. The viscosity of argon is 1.21, referred to air,
somewhat higher than for oxygen, which stands at the head of the list of
the principal gases ("On some Physical Properties of Argon and Helium,"
_Proc. Roy. Soc._ vol. lix. p. 198, 1896).
The spectrum shows remarkable peculiarities. According to circumstances,
the colour of the light obtained from a Plucker vacuum tube changes
"from red to a rich steel blue," to use the words of Crookes, who first
described the phenomenon. A third spectrum is distinguished by J.M.
Eder and Edward Valenta. The red spectrum is obtained at moderately low
pressures (5 mm.) by the use of a Ruhmkorff coil without a jar or
air-gap. The red lines at 7056 and 6965 (Crookes) are characteristic.
The blue spectrum is best seen at a somewhat lower pressure (1 mm. to
2.5 mm.), and usually requires a Leyden jar to be connected to the
secondary terminals. In some conditions very small causes effect a
transition from the one spectrum to the other. The course of electrical
events attending the operation of a Ruhmkorff coil being extremely
complicated, special interest attaches to some experiments conducted by
John Trowbridge and T.W. Richards, in which the source of power was a
secondary battery of 5000 cells. At a pressure of 1 mm. the red glow of
argon was readily obtained with a voltage of 2000, but not with much
less. After the discharge was once started, the difference of potentials
at the terminals of the tube varied from 630 volts upwards.
The introduction of a capacity between the terminals of the Geissler
tube, for example two plates of metal 1600 sq. cm. in area separated
by a glass plate 1 cm. thick, made no difference in the red glow so
long as the connexions were good and the condenser was quiet. As soon
as a spark-gap was introduced, or the condenser began to emit the
humming sound peculiar to it, the beautiful blue glow so
characteristic of argon immediately appeared. (_Phil. Mag._ xliii. p.
77, 1897.)
The behaviour of argon at low temperatures was investigated by K.S.
Olszewski (_Phil. Trans._, 1895, p. 253). The following results are
extracted from the table given by him:--
+----------+--------------+-----------+----------+----------+
| | Critical | Critical | Boiling | Freezing |
| Name. | Temperature, | Pressure, | Point, | Point, |
| | Cent. | Atmos. | Cent. | Cent. |
+----------+--------------+-----------+----------+----------+
| Nitrogen | -146.0 | 35.0 | -194.4 | -214.0 |
| Argon | -121.0 | 50.6 | -187.0 | -189.6 |
| Oxygen | -118.8 | 50.8 | -182.7 | ? |
+----------+--------------+-----------+----------+----------+
The smallness of the interval between the boiling and freezing points is
noteworthy.
From the manner of its preparation it was clear at an early stage that
argon would not combine with magnesium or calcium at a red heat, nor
under the influence of the electric discharge with oxygen, hydrogen or
nitrogen. Numerous other, attempts to induce combination also failed.
Nor does it appear that any well-defined compound of argon has yet been
prepared. It was found, however, by M.P.E. Berthelot that under the
influence of the silent electric discharge, a mixture of benzene vapour
and argon underwent contraction, with formation of a gummy product from
which the argon could be recovered.
The facts detailed in the original memoir led to the conclusion that
argon was an element or a mixture of elements, but the question between
these alternatives was left open. The behaviour on liquefaction,
however, seemed to prove that in the latter case either the proportion
of the subordinate constituents was small, or else that the various
constituents were but little contrasted. An attempt, somewhat later, by
Ramsay and J. Norman Collie to separate argon by diffusion into two
parts, which should have different densities or refractivities, led to
no distinct effect. More recently Ramsay and M.W. Travers have obtained
evidence of the existence in the atmosphere of three new gases, besides
helium, to which have been assigned the names of neon, krypton and
xenon. These gases agree with argon in respect of the ratio of the
specific heats and in being non-oxidizable under the electric spark. As
originally defined, argon included small proportions of these gases, but
it is now preferable to limit the name to the principal constituent and
to regard the newer gases as "companions of argon." The physical
constants associated with the name will scarcely be changed, since the
proportion of the "companions" is so small. Sir William Ramsay considers
that probably the volume of all of them taken together does not exceed
1/400th part of that of the argon. The physical properties of these
gases are given in the following table (_Proc. Roy. Soc._ lxvii. p. 331,
1900):--
+----------------+---------+--------+--------+----------+--------+
| | Helium. | Neon. | Argon. | Krypton. | Xenon. |
+----------------+---------+--------+--------+----------+--------+
| Refractivities | .1238 | .2345 | .968 | 1.449 | 2.364|
| (air = 1) | | | | | |
| Densities | 1.98 | 9.97 | 19.96 | 40.88 | 64 |
| (O = 16) | | | | | |
| Boiling points | c. 6 | ? | 86.9 | 121.33 | 163.9 |
| at 760 mm. | deg.[1]| | deg. | deg. | deg. |
| | abs. | | abs. | abs. | abs. |
| Critical | ? | below |155.6 | 210.5 | 287.7 |
| temperatures | | 68 deg.| deg. | deg. | deg. |
| | | abs. | abs. | abs. | abs. |
| Critical | ? | ? | 40.2 | 41.24 | 43.5 |
| pressures | | | metres.| metres. | metres.|
| Weight of 1 | ? | ? | 1.212 | 2.155 | 3.52 |
| c.c. of liquid| | | gm. | gm. | gm. |
+----------------+---------+--------+--------+----------+--------+
The glow obtained in vacuum tubes is highly characteristic, whether as
seen directly or as analysed by the spectroscope.
Now that liquid air is available in many laboratories, it forms an
advantageous starting-point in the preparation of argon. Being less
volatile than nitrogen, argon accumulates relatively as liquid air
evaporates. That the proportion of oxygen increases at the same time is
little or no drawback. The following analyses (Rayleigh, _Phil. Mag._,
June 1903) of the _vapour_ arising from liquid air at various stages of
the evaporation will give an idea of the course of events:--
+---------------+---------------+--------------------------+
| Percentage of | Percentage of | Argon as a Percentage of |
| Oxygen. | Argon. | the Nitrogen and Argon. |
+---------------+---------------+--------------------------+
| 30 | 1.3 | 1.9 |
| 43 | 2.0 | 3.5 |
| 64 | 2.0 | 5.6 |
| 75 | 2.1 | 8.4 |
| 90 | 2.0 | 20.0 |
+---------------+---------------+--------------------------+
(R.)