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the late Professor Lothar Meyer, and the table
may be made to assume the subjoined form (the

THE ELEMENTS ARRANGED IN



Lithium . 7'0


Beryllium . 9'1


Boron . . ll'O


Carbon. . 12'0


Sodium . .23-0


Magnesium 24 -4


Aluminium 27*1


Silicon . . 28-4


Potassium .391


Calcium . 40*0


Scandium . 44 '1


Titanium 43-1


Rubidium . 85 '4


Strontium . 87 '6


Yttrium . 89 '0


Zirconium 90 -0


Caesium 133 '0


Barium . . 137*4


Lanthanum 138 '0


Cerium . 140 '3


? . . . . 170-0


? . . . . 172-0


Ytterbium 173'0


? . . . 177-0


? . . . . 221-0


Radium . 225-0


? . . . . 230-0


Thorium 232*4



VII



ARGON AND OTHER ELEMENTS



221



atomic weights are given with only approximate
accuracy) :

The elements in the first column all agree
in that they are white soft substances, with
metallic lustre, but tarnish rapidly in air, owing
to the action of water - vapour ; they are all
violently attacked by water, and they are with-
out exception monads, that is, they replace
hydrogen in its compounds atom for atom. The
elements in column two are also all white
metals, attacked by water with more or less
ease ; but in their case one atom replaces
two atoms of hydrogen, whence they are called
dyads, or bivalent elements (worth two). And



THE PERIODIC SYSTEM.







Hydrogen . I'Ol


Helium . 4'0


Nitrogen . 14'0


Oxygen . 16'0


Fluorine . 19'0


Neon . . 20-0


Phosphorus 31 '0


Sulphur . 32-1


Chlorine . 35*5


Argon . . 39-9


Vanadium . 51-4


Chromium . 52'3 Manganese . 55'0


riron . ."56-0
{ Cobalt . . 587








(Nickel. . 58-6


Niobium . 94-0


Molybdenum 957


' . . 100-0


( Ruthenium 101-6
-j Rhodium . 103 '0


Neodymium 140-8


Praseodymium 143 -6


Samarium . 150'0


(Palladium 106 -3
? 152, 153, 154


Tantalum . 182-5


Tungsten . 184*0


! * . . . 190-0


( Osmium . 191-3
j Iridium . 193-0


?

_


Uranium . 240*0

__


? . . . .244-0

, _


(Platinum . 194-3

.



222 THE GASES OF THE ATMOSPHERE CHAP.

so on with the other columns. All elements in
vertical columns exhibit chemical similarity, and,
indeed, are often strikingly like in properties.

The subdivision, produced by folding the loose
slip, is intended to show that the elements repre-
sented on it have a double set of resemblances.
But there are various anomalous and inexplicable
phenomena still attached to this arrangement of
elements. For example, copper, although it replaces
one atom of hydrogen in some of its compounds,
and is thus a monad, forms more numerous and
more stable compounds in acting as a dyad and
replacing two atoms of hydrogen. Gold, which
belongs to the same column, is at once univalent
and tervalent ; mercury, both univalent and bi-
valent ; thallium, univalent and tervalent ; tin and
lead, bivalent and quadrivalent, and so on. It is as
if some elements had a tendency to enter a column
not their own.

Again, on comparing the atomic weights of the
elements, it is seen that the differences are far from
being regular. As a rule, the difference in the
vertical columns between any single element and
the one following it is approximately 16, or some
multiple of 16. Thus we have lithium, sodium,



vii ARGON AND OTHER ELEMENTS 223

and potassium; beryllium, magnesium, and cal-
cium; boron, aluminium, and scandium ; carbon
and silicon; oxygen and sulphur; fluorine and
chlorine all with a difference of 16 approximately.
But here we come to a break : silicon and titanium,
phosphorus and vanadium, sulphur and chromium,
chlorine and manganese, each show a difference
of about 20.

Passing on, between the atomic weights of
potassium, rubidium, and caesium there is a differ-
ence of about 16x3; a similar difference between
calcium, strontium, and barium ; between scandium
and yttrium ; between titanium, zirconium, and
cerium, and so on ; but with wider and wider
divergence from the supposed constant, 48 = 16 x 3.
In short, we have a seeming regularity, but only
a very approximate one a regularity, in fact, in
which a vivid imagination must play a conspicuous
part in order to detect it.

Now, up to the present, no reason has been
suggested to account for the divergence from this
irregular regularity, which a little expenditure of
time will enable any one to trace through all these
numbers. But one thing has been remarked :
there is the same seeming regularity between



224 THE GASES OF THE ATMOSPHERE CHAP.

certain physical properties of elements and their
compounds : their specific volumes, their melting-
points, their refractive indices, and other properties
vary from member to member of the same column
in a manner bearing more or less similarity to the
periodic variation of the atomic weights.

It happens that among compounds of carbon
we are acquainted with series of compounds
which, in variation of molecular weights and grada-
tion of properties, bear a striking resemblance to
the elements thus arranged. Thus we have the

series :

CH 4 Methane 16

C 2 H 6 Ethane 30

C 3 H 8 Propane 44

C 4 H 10 Butane 58

C 6 H 12 Pentane 72

and a host of others up to a compound of the
formula C 30 H 62 ; in each case there is a constant
difference of 14 between the molecular weight
of any one hydrocarbon and that immediately
preceding or succeeding it in the column. Such a
series is termed a homologous series. The analogy
is very tempting ; to suppose that a similar con-
stant difference should exist in the relations of the
atomic weights of the elements, and that they too



vii ARGON AND OTHER ELEMENTS 225

are undecomposable compounds of two unknown
elements, is an attractive hypothesis, but one
for which there exists no proof; indeed, it is
rendered improbable by the irregularities just
pointed out.

But there is one noticeable feature in the
periodic arrangement of the elements. It is, that
although the differences are irregular (e.g. between
B = 11 and C = 12 the difference is 1, while between
= 16 and F = 19 the difference is 3), yet there are
two marked displacements in the order of arrange-
ment of the elements, inasmuch as two elements
have atomic weights lower than those preceding
them in the horizontal line. It is tellurium and
iodine, and nickel and cobalt which are thus mis-
placed ; and the same is evidently the case with
potassium and argon. With an atomic weight of
3 9 '8 8, the natural position of argon would lie
between potassium and calcium ; but there is no
room for it. And for this reason considerable
doubt has been thrown on the validity of the
conclusion to be drawn from the found ratio of
its specific heats, If, viz. that its molecule and
its atom are identical. If it were a diatomic
gas, like chlorine or hydrogen, its atomic weight



226 THE GASES OF THE ATMOSPHERE CHAP.



would be 19 '94, and it would find a fitting-
position after fluorine and before sodium. And the
difference between its atomic weight and that of
helium, to which the atomic weight 2'0 would for
the same reasons then attach, would be 17*94, one
not incomparable with 16. But, as before re-
marked, it is difficult, if not altogether impossible,
to conceive of a diatomic structure to which all
energy imparted in the form of heat should result
in translational motion, and as a matter of fact
none such is known.

There are two methods of escape from this
dilemma. If the gases termed argon and helium
are not single elements, but mixtures of monatomic
elements, then what has been termed their atomic
weights will represent the mean of the atomic
weights of two or more elements, taken in the
proportion in which they occur. For example,
supposing that argon is a mixture of an element of
atomic weight 37 with one of atomic weight 82,
the found atomic weight, nearly 40, would imply a
mixture of 93*3 per cent of the lighter, with 6*7 per
cent of the heavier element. We must therefore
carefully examine all evidence for or against the
supposition that argon is a mixture of elements.



vii ARGON AND OTHER ELEMENTS 227

It is well known that elements with high
atomic weights have, as a rule, higher boiling-
points than those with low atomic weights in the
same columns. Perhaps the most striking case is
that of the elements fluorine, chlorine, bromine,
and iodine. Whereas fluorine has only lately been
liquefied (chiefly owing to difficulties of manipula-
tion, due to its extraordinarily energetic action
on almost every element and compound), and boils
at about - 185, chlorine boils at - 34, bromine at
59, and iodine at 184. And if a mixture of
chlorine and bromine gases be cooled, the bromine,
if present in sufficient amount, will condense first,
in a fairly pure state, little chlorine condensing
with it. But in a mixture containing only 7
per cent of bromine with 93 per cent of chlorine
(analagous to a mixture of the two supposed
constituents of the argon mixture) the pressure
of the bromine gas in the mixture would be
only y-J^ths of the normal pressure, or 53*2
millimetres. At this pressure the boiling-point
of bromine is about -5, so that, on cooling to
that temperature, bromine would begin to show
signs of liquefaction. This is, however, still nearly
30 above the boiling-point of chlorine ; and there



228 THE GASES OF THE ATMOSPHERE CHAP.

would therefore be no difficult} 7 whatever in detect-
ing such a percentage of bromine in a mixture of
chlorine and bromine gases on cooling the mixture
to a moderately low temperature. 1

Argon was first liquefied in 1895. A sample
of pure argon was sent by Professor Eamsay to
Professor Olszewski of Cracow, well known for his
accurate researches at low temperatures ; and he
found the boiling-point of argon at atmospheric
pressure to be 186 '9, and its melting-point to
be 189*6. 2 There was no appearance of liquid
before the boiling-point was reached, nor was there
any alteration of temperature as the argon boiled
away, and these are signs of a single substance,
not of a mixture ; moreover, the melting-point was
a definite one; and here again, mixtures never
melt suddenly, but always show signs of softening
before melting. So far as this evidence goes,
therefore, it points to the conclusion that argon is
not a mixture of elements.

Other evidence may be sought for in the
spectrum of argon, which has been carefully ex-

1 These considerations would hold on the assumption that no com-
bination takes place between chlorine and bromine.

2 Later determinations by Ramsay and Travers have altered these
numbers to - 186 '1 and - 187 '9 (FluL Trans. 1901, A, p. 41).



vii ARGON AND OTHER ELEMENTS 229



amined by Sir William Crookes and others. It
consists of a great number of lines, extending all
through the spectrum, from far down in the red to
far beyond the visible violet ; the invisible lines
were examined by the aid of photography, for
ultra-violet light, although invisible to the eye,
impresses a photographic plate. The most striking
feature of this spectrum is the change which can
be produced in it by altering the intensity of the
electric discharge which is passed through the tube
containing argon at a low pressure. By interposing
a Leyden jar between the secondary terminals of
the induction-coil from which sparks are taken
through the gas, the colour of the light in the tube
changes from a brilliant red to an equally brilliant
blue. A large number of lines in the red spectrum
disappear, on interposing the jar, while many lines
in the blue- green, blue, and violet part of the
spectrum, invisible before, shine out with great
brilliancy. There is no other gas in which a
similar alteration of intensity of 'discharge produces
such a marked difference, although in many gases,
supposed to be simple substances, similar changes
may be produced. So far as we know at present,
however, such a change cannot be definitely



230 THE GASES OF THE ATMOSPHERE CHAP.

ascribed to the presence of a mixture of two
elements, although it is in itself a very remarkable
phenomenon.

On the other hand, Professors Runge and
Paschen, in a paper communicated to the Koyal
Academy of Science of Berlin in July 1895,
adduced reasons for concluding that helium, the
gas from cleveite, is a mixture ; it appears to show
lines belonging to two spectra, each series of lines
exhibiting certain regularities. But this is also
the case with oxygen, which is not considered to
be a mixture of gases.

One method of separating the constituents of a
mixture is by taking advantage of their different
solubilities in water, or in some other appropriate
solvent. And as argon was found to have the
solubility of 4 volumes in 100 of water, while
helium is more sparingly soluble, only 1 *4 volume
per 100, it is not unreasonable to suppose that,
if argon consisted of a mixture of elements in
argon, one should be more soluble than another ;
but Lord Eayleigh has made experiments which
render it very improbable that any separation of
its constituents can be thus effected. Wishing to
ascertain if there were any helium in the air, he



vir ARGON AND OTHER ELEMENTS 231

shook up atmospheric argon with water, until a
very small fraction remained undissolved. The
spectrum of this small residue was identical with
that of the original argon, from which it would
appear that this method, at least, did not effect
any separation.

The evidence was therefore at first distinctly
against the supposition that argon is a mixture of
two or more elements. .

There is, however, another possible method of
accounting for the high a,tomic weight of argon,
which, if it could be reduced by a few units, would
fall into its place after chlorine and before
potassium. It is that argon consists of a mixture
of many mon atomic, with comparatively few dia-
tomic, molecules. If there were only about 500
molecules of diatomic argon in every 10,000
molecules of the gas, its density, supposing it to
consist entirely of monatomic molecules, would be
19, and its atomic and molecular weights 38 a
number which would fit between the atomic weight
of chlorine, 3 5 '5, and that of potassium, 39'1.
Several instances of this kind are known. Chlorine
itself, when heated to high temperatures, changes
from diatomic to monatomic molecules, and the



232 THE GASES OF THE ATMOSPHERE CHAP.



density decreases with the change. For example,
at 1000 the found density of chlorine is 27,
implying a molecular weight of 54 ; now 54 is
neither the weight of a monatomic molecule of
chlorine, viz. 35 '5, nor of a diatomic molecule,
which is 71 ; but it corresponds to that of a mix-
ture of monatomic and diatomic molecules. Here
fall of temperature causes combination of monatomic
molecules with each other to form diatomic mole-
cules ; and rise of temperature increases the number
of monatomic molecules, at the expense of the
diatomic molecules. Is there no sign of similar
behaviour with argon ?

It has already been mentioned that the rise of
pressure of argon with rise of temperature has been
carefully measured by Drs. Eandal and Kuenen,
and that it is quite normal ; no sign of splitting
has been observed. But the range of temperature
was not great (it was only from to 280), and it
is quite possible that the change, if there was one,
was so minute as to have escaped detection. Again,
a more delicate method of detecting such a change
is in the measurement of the ratio of the specific
heats. The most trustworthy number obtained was
1*659 for the ratio, instead of 1*667, the theoretical



vii ARGON AND OTHER ELEMENTS 233

figure. A mixture of 5 per cent of diatomic mole-
cules should have reduced this ratio to 1*648.
Here the evidence is, however, inconclusive. But
on the whole, the presumption is against the
hypothesis that argon is a mixture of monatomic
with diatomic molecules.



Q



CHAPTER VIII

THE OTHER INACTIVE GASES I HELIUM, NEON,
KRYPTON, AND XENON

IN 1868 an eclipse of the sun was visible in India.
The spectroscope was then for the first time em-
ployed to examine the chromosphere, or coloured
atmosphere round the sun ; and a brilliant yellow
line was observed, and supposed to be the " D "
lines of sodium. The well-known French astronomer,
M. Janssen, however, noted its non- coincidence
with those lines ; and it was supposed to be due to
hydrogen, or to water-vapour ; but all attempts to
reproduce the line in the laboratory failed. Messrs.
Lockyer and Frankland, who investigated the
chromosphere spectrum, found that the line which
was distinguished as "D 3 " could not be ascribed
to the spectrum of any known element \ and as a

matter of convenient reference, Frankland suggested

234



CHAP, viii THE OTHER INACTIVE GASES 235

the provisional name of "helium," a name derived
from the Greek word for the sun, ^Xio?.

No certain proof that this supposed element
existed on the earth was obtained, although a
specimen of gas from a mud- volcano near Vesuvius
was said to have exhibited the line.

In seeking for compounds of argon, Professor
Ramsay was reminded by Professor Miers that
Dr. Hillebrand, of the U.S. Geological Survey, had
obtained relatively considerable volumes of gas,
which was supposed to be nitrogen, by heating a
rare mineral named cleveite, after Cleve, the Pro-
fessor of Chemistry at Upsala. Of course, if a
substance were found, from which argon could be
obtained on heating, it would give a clue to the
elements with which an attempt to cause argon to
combine might be successful. A sample of cleveite
was procured, and heated with sulphuric acid ; and
a gas was collected, which, after purification by
sparking with oxygen in presence of caustic soda,
was examined with the spectroscope. The chief
characteristic of the spectrum was a brilliant yellow
line, much overpowering the others in intensity ;
and the first idea was that it must have been due
to the discharge making the soda in the glass of



236 THE GASES OF THE ATMOSPHERE CHAP.

the vacuum-tube incandescent. The position of
the line was not coincident, however, with that of
the sodium lines thrown into the field of vision for
the purpose of comparison ; the preconceived idea
that the line was due to sodium was hard to
eradicate ; and the spectroscope was dismantled,
the prisms readjusted, and the spectra again com-
pared. This time there could be no doubt ; the
lines were not coincident. Reference to a table of
the solar spectrum soon made the matter clear,
and terrestrial helium was discovered. 1 Like argon,
it is a gas, with no pronounced tendency towards
combination ; it is, like argon, nearly insoluble in
water; while 100 volumes of water at atmospheric
temperature (15 C.) dissolve 4*1 volumes of argon,
they dissolve only 1 *4 volumes of helium ; for the
solubility of helium' is nearly the same as that of
nitrogen, the least soluble of gases. Attempts
made to induce helium to enter into combination
failed, like those made with argon ; and it is there-
fore reasonable to place it in the same class of
elements as argon, especially as the ratio between
its specific heats shows it to resemble argon in

1 It was somewhat later, but independently, rediscovered by Langlet,
in Cleve's laboratory.



vin THE OTHER INACTIVE GASES 237

being a monatomic gas. Its density is nearly 2'0
that of oxygen being taken at 16'0; next to
hydrogen, the density of which is 1*007, it is the
lightest gas known.

It was subsequently found that many minerals,
chiefly those which contain the rare element
uranium (the element of highest atomic weight
known), contain helium, and give it off when
heated ; among these are broggerite, fergusonite
(which turns white-hot during the evolution of
helium), and monazite, a mineral now mined in
large quantity in Brazil, and used as a source of
the thoria of which incandescent gas-mantles are
made. The curious behaviour of fergusonite de-
serves explanation.

It is familiar to all that a burning object, such
as coals, or a candle, gives out heat. This is
generally the case when chemical combination
takes place. Even when chlorine combines with
elements such as iron or phosphorus, heat is
evolved. Now, there are compounds which are
formed not with evolution of heat, but with absorp-
tion. And such substances give off heat when
they decompose ; indeed they usually decompose
with explosion. Gun-cotton and the newer forms



238 THE GASES OF THE ATMOSPHERE CHAP.

of smokeless powders are instances in point ;
another is acetylene gas, which owes its high
luminosity to the great heat given off when it
decomposes ; this heat adds itself to that evolved
by the burning of the carbon and hydrogen of
which it consists, and the particles of carbon which
separate in the flame are raised to brighter incan-
descence than if they owed their temperature to
the heat due to the burning of the carbon and
hydrogen alone, as is the case with ordinary coal-
gas. Compounds which behave thus are termed
" endothermic," and fergusonite is an endothermic
compound. Now it is found that endothermic
compounds are not readily produced from their
elements ; for chemical combination takes place
generally only when heat is evolved during com-
bination when the reaction is " exothermic." But
it is possible to produce endothermic compounds
directly ; that can be achieved if energy is given
them during combination. For example, nitrogen
does not burn in oxygen ; if it did, our atmosphere
would be inflammable. But if an electric dis-
charge be passed through a mixture of nitrogen
and oxygen, combination does occur : that is one
of the methods employed for removing nitrogen



viii THE OTHER INACTIVE GASES 239

from a mixture of that gas with argon or helium.
Now, the fact that fergusonite decomposes with
evolution of heat implies that the helium which it
evolves when heated must have entered into com-
bination with the constituents of the mineral with
absorption of heat, or that some rearrangement of
molecules takes place on heating.

That helium is a pretty abundant constituent
of the earth is proved by its being contained in
many mineral waters. The springs at Cauterets,
in the Pyrenees, evolve it in fair quantity ; and,
nearer home, the mineral waters of Bath are rich
in it. It doubtless escapes from the soil in many
places ; and, as will hereafter appear, it is a con-
stituent of our atmosphere. Like argon, helium
has neither taste nor smell ; indeed, its inactive
character would have rendered this probable, even
without direct evidence. It also resembles argon
in being a monatomic gas ; for the ratio of its
specific heats is 1 to If , as explained on p. 203 ;
and with such a ratio, the atom and the molecule
are identical.

The spectrum of helium is a very brilliant one.
Besides the particularly brilliant yellow line, by
means of which it was originally recognised as the



240 THE GASES OF THE ATMOSPHERE CHAP.

sun's chromosphere and in many of the fixed stars,
it exhibits two red lines, of which one is fairly
brilliant ; also, besides other fainter ones, a green
line, a peacock-blue line, and a violet line. It was
at first conjectured that what was named helium
was a mixture of two gases, and, indeed, the name
" parhelium " or " asterium " was given to the
supposed partner ; but this supposition was found
to be erroneous, for the arguments in its favour
(namely, that the various lines in its spectrum
could be arranged in two series, the lines in each
series exhibiting numerical relations to each
other, if arranged according to the frequency
of vibration of the light-waves) were shown to
apply equally to oxygen ; and it is not believed
that oxygen is a compound gas. Professors Kayser
and Eunge, who made this suggestion, afterwards
disproved it ; and another suggested argument,
which was that diffusing the gas through porous
pipe-clay separated a light from a heavy portion,
the one giving a more brilliant green, and the
other a more brilliant yellow line, also turned out
to be inaccurate ; as a matter of fact, if the pressure
in a vacuum-tube containing helium is reduced,
the yellow line is relatively weakened in intensity,



viii THE OTHER INACTIVE GASES 141

while the green line grows stronger and more
luminous. Careful experiments, by which helium
was fractionally diffused many hundred times, also
proved the homogeneity of helium. The spectrum
of helium, too, like that of argon, is altered by
the interposition of a jar and a spark-gap ; but the
change is by no means so striking as with argon.

After the discovery of helium, it appeared cer-
tain that other gases remained to be discovered,
similar to those which had already been isolated.


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