THE POSITION OF ARGON AMONG THE ELEMENTS
From what has been said in the preceding chapter there can be no doubt
that the molecular weight of argon is 39·88. We have now to consider
what this conclusion involves. Taken in conjunction with the fact that
the ratio between its specific heat at constant volume and that at
constant pressure is 1⅔, it follows that energy imparted to it is
employed solely in communicating translational motion to its molecules.
In the case of mercury gas such behaviour is taken as evidence that
the conclusion following from the formulae of its compounds, from
the density of its compounds in the gaseous state, and from its own
vapour-density, as well as from its specific heat in the liquid state,
namely, that its molecules are monatomic, is correct. Is it legitimate
to conclude that because argon in the gaseous state has the same ratio
of specific heats, therefore it also is a monatomic gas?
The conclusion will depend on our conception of an atom and a molecule,
and in the present state of our ignorance regarding these abstract
entities no positive answer can be given. It appears certain that,
on raising the temperature of argon, very little, if any, energy is
absorbed in imparting vibrational motion to its molecules; and our
choice lies between our ability or inability to conceive of a molecule
so constituted as to be incapable of internal motion. If there be any
truth underlying Professor Boltzmann’s conception, a molecule of argon
cannot consist of any complex structure of atoms, otherwise it would
possess more than three degrees of freedom, and heat would be utilised
in causing rotational motions. As we know for a fact that the ratio
between the specific heats of gases diminishes with the increasing
complexity of their molecules, perhaps the safest conclusion is the one
adopted by the discoverers of argon, that the balance of evidence drawn
from _this_ source is in favour of its monatomic nature.
But this hypothesis raises difficulties which are not lightly to be
met. These difficulties arise from a consideration of the position of
argon when it is classified with other elements.
In 1863 Mr. John Newlands pointed out in a letter to the _Chemical
News_ that if the elements be arranged in the order of their atomic
weights in a tabular form, they fall naturally into such groups that
elements similar to each other in chemical behaviour occur in the
same columns. This idea was elaborated farther in 1869 by Professor
Mendeléeff of St. Petersburg and by the late Professor Lothar Meyer,
and the table may be made to assume the subjoined form (the atomic
weights are given with only approximate accuracy):--
THE ELEMENTS ARRANGED IN THE PERIODIC SYSTEM.
+---------------+---------------+----------------+---------------
| | | |
| | | |
|Lithium 7·0|Beryllium 9·1|Boron 11·0|Carbon 12·0
| | | |
|Sodium 23·0|Magnesium 24·3|Aluminium 27·0|Silicon 28·3
| | | |
|Potassium 39·1|Calcium 40·1|Scandium 44·1|Titanium 48·1
| | | |
|Rubidium 85·5|Strontium 87·5|Yttrium 89·0|Zirconium 90·0
| | | |
|Caesium 132·9|Barium 137·0|Lanthanum 142·3|Cerium 140·3
| | | |
|? 170·0|? 172 0|Ytterbium 173·0|? 177·0
| | | |
|? 221·0|? 225·0|? 230·0|Thorium 232·4
+---------------+---------------+----------------+---------------
----------------+----------------+--------------+-----------+
| |Hydrogen 1·01|Helium 4·2|
| | | |
Nitrogen 14·0|Oxygen 16·0|Fluorine 19·0|? |
| | | |
Phosphorus 31·0|Sulphur 32·1|Chlorine 35·5|Argon 39·9|
| | | |
Arsenic 75·1|Selenium 79·0|Bromine 80·0|? |
| | | |
Antimony 120·3|Tellurium 126·3|Iodine 126·9|? |
| | | |
Erbium 166·0|? 167·0|? 169·0|? |
| | | |
Bismuth 208·1|? 214·0|? 219·0|? |
| | | |
| | | |
----------------+----------------+--------------+-----------+
+-------------+---------------+---------------+---------------
| | | |
| | | |
|Lithium 7·0|Beryllium 9·1|Boron 11·0|Carbon 12·0
| | | |
|Sodium 23·0|Magnesium 24·3|Aluminium 27·0|Silicon 28·3
| | | |
| | | |
|Copper 63·4|Zinc 65·3|Gallium 69·9|Germanium 72·3
| | | |
| | | |
| | | |
|Silver 107·9|Cadmium 112·1|Indium 113·7|Tin 119·1
| | | |
| | | |
|? 156·0|? 158·0|? 159·0|Terbium 162·0?
| | | |
| | | |
|Gold 197·2|Mercury 200·2|Thallium 204·2|Lead 206·9
| | | |
| | | |
| | | |
+-------------+---------------+---------------+---------------
---------------+------------------+--------------+----------------+
| |Hydrogen 1·01|Helium 4·2|
| | | |
Nitrogen 14·0|Oxygen 16·0|Fluorine 19·0|? |
| | | |
Phosphorus 31·0|Sulphur 32·1|Chlorine 35·5|Argon 39·9|
| | | |
| | |{Iron 56·0|
Vanadium 51·4|Chromium 52·3|Manganese 55·0|{Cobalt 58·7|
| | |{Nickel 58·6|
| | | |
| | |{Ruthenium 101·6|
Niobium 94·0|Molybdenum 95·7|? 100·0|{Rhodium 103·0|
| | |{Palladium 106·3|
| | | |
Neodymium 140·8|Praseodymium 143·6|Samarium 150·0|? 152, 153, 154 |
| | | |
| | |{Osmium 191·3|
Tantalum 182·5|Tungsten 184·0|? 190·0|{Iridium 193·0|
| | |{Platinum 194·3|
| | | |
? |Uranium 240·0|? 244·0| |
---------------+------------------+--------------+----------------+
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 without 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 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 page, is intended
to show that the elements represented 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 bivalent; 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, and potassium; beryllium, magnesium, and calcium;
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 difference of about 16 × 3; 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 × 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
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 gradation 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_{5}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
constant difference should exist in the relations of the atomic weights
of the elements, and that they too 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 O = 16
and F = 19 the difference is 3), yet there is no marked displacement
in the _order_ of arrangements of the elements, inasmuch as no element
has an atomic weight _lower_ than that preceding it in the horizontal
line. It was for some time supposed that tellurium and iodine were thus
misplaced; and indeed it is even now not quite established that they
are not, but the balance of evidence is in favour of tellurium having a
lower atomic weight than iodine.
Argon, however, is a marked exception. With an atomic weight of 39·88,
its natural position would lie between those of potassium and calcium;
but there is no room for it. And for this reason considerable doubts
have been thrown on the validity of the conclusion to be drawn from the
found ratio of its specific heats, 1⅔, viz. that its molecule and
its atom are identical. If it were a diatomic gas, like chlorine or
hydrogen, its atomic weight 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
2·1 would for the same reasons then attach, would be 17·84, one not
incomparable with 16. But, as before remarked, 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.
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 never
been liquefied (chiefly owing to difficulties of manipulation, due
to its extraordinarily energetic action on almost every element and
compound), chlorine boils at -102°, 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
(analogous to a mixture of the two supposed constituents of the argon
mixture) the pressure of the bromine gas in the mixture would be only
7/100ths 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 100° above the boiling-point of chlorine;
and there would therefore be no difficulty whatever in detecting such
a percentage of bromine in a mixture of chlorine and bromine gases on
cooling the mixture to a moderately low temperature.[30]
Argon has been liquefied. A sample of pure argon was sent by Professor
Ramsay 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°. 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 two elements.
Other evidence may be sought for in the spectrum of argon, which was
carefully examined by Mr. Crookes. 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 ultraviolet 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 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 Royal Academy of Science of Berlin in July 1895,
have adduced reasons for concluding that helium, the gas from clèveite,
is a mixture; it appears to show lines belonging to two spectra, each
series of lines exhibiting certain regularities. But this, although an
important conclusion in itself, has no direct bearing on the question
of the simplicity of argon.
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 very sparingly soluble, only
0·7 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. Exhaustive experiments in this direction have still to
be carried out; but Lord Rayleigh has made experiments which render it
very improbable that any separation into its constituents, if it be a
mixture, can be thus effected. Wishing to ascertain if there were any
helium in the air, he 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, is incapable of effecting any
separation.
A completely decisive proof that argon is not a mixture has just been
furnished by experiments carried out by Dr. Collie and Professor
Ramsay, in which a large quantity of argon was submitted to fractional
diffusion. From what was said on p. 162, it will be seen that if argon
consisted of a mixture of two gases of different densities, such a
process should separate the mixture more or less completely into its
two constituents. After a long series of diffusions, however, the
density of that portion of argon passing first through the porous plug,
which would have been less had any gas of lower density been present,
was found to be identical with that of the last portions of gas. On the
other hand, by aid of the same diffusion-apparatus, a fair separation
of oxygen (density 16) from carbon dioxide (density 22) was effected,
although, as the reader will observe, the densities of these two gases
do not differ greatly. Hence, if argon consists of two kinds of matter
they must have the same density, and hence the same molecular weights,
and the difficulty is not removed. But as the spectrum of the first and
last portions was the same and was identical with that of argon, this
supposition is improbable.
The evidence is therefore 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
atomic 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 monatomic, with comparatively
few diatomic, 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, 35·5, 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 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 mixture of monatomic and diatomic molecules.
Here fall of temperature causes combination of monatomic molecules
with each other to form diatomic molecules; 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. Randall 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
0° 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 figure. A mixture of
5 per cent of diatomic molecules 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.
It still remains for us, therefore, to account for the fact that in the
periodic table there is no place for argon, provided it be insisted
on that the elements must follow each other in the numerical order
of their atomic weights. If the numbers in the table actually showed
regular intervals, or if there were any regularity to be detected in
their differences, argon might be regarded as of wholly exceptional
behaviour. But this is not so. Argon is an extreme instance of
divergence, but similar divergences, though not of equal magnitude, are
common.
In attempting to offer an explanation of such anomalies, it must be
remembered that the question is in itself a far-reaching one; and that
although argon has served to direct attention anew to the anomalies
of the periodic table, yet these anomalies existed before argon was
discovered. It is necessary above all things to be clear as to what is
under discussion. We speak of “atomic weights,” or “atomic masses.”
What is meant precisely by these expressions?
By mass, we understand that property of a body, in virtue of which,
when acted on by a certain force for a certain time, it acquires a
certain velocity. The product of the mass into half the velocity
squared, or ½MV^2 (where M and V stand respectively for mass and for
velocity), is what is termed kinetic energy. If the mass chosen be 1
gram, and the velocity 1 centimetre per second, the unit of energy
is the product; it is termed an erg. The same unit of energy, the
erg, is derived by the action of unit force, termed 1 dyne, through
unit length, 1 centimetre. We have thus two equations, where F and L
represent force and length,
Kinetic Energy = ½(MV)^2 and Linear Energy = FL.
Now we choose mass as a unit of measure of the _quantity of material_;
and we are justified in doing so, because experiments have shown that
material, confined in a closed space, does not appreciably alter its
mass. The mass is proportional to the _weight_, generally measured as
the force exerted at some definite latitude on the earth’s surface,
tending to pull the body towards the centre of the earth. This force
is equal to about 981 dynes at London. There is no known reason
why mass and weight should be proportional to each other; for the
cause of the attraction of the earth has never been satisfactorily
elucidated. We therefore use weight, or the attraction of the earth, as
a convenient means of determining the relative masses of two objects.
Hence it appears rational to prefer the expression “atomic weight” to
“atomic mass,” seeing that the former represents the actual result of
experiment, and also because we are dealing in atomic weights with
relative numbers. But this is really a matter of choice.
The atomic weights therefore represent the relative masses in which
the elements generally unite. They often, however, unite in multiples
of these weights, as formulated by Dalton’s second law. The weights,
arranged in numerical order in columns, give us the periodic table.
Now energy can be measured in other units besides those of force and
mass. Heat is one form of energy, and it is measured by an interval of
temperature, and by a property which we term specific heat. It happens
that the latter property varies, not with the mass of the substance
heated, but with its atom, so that all elements have approximately
the same atomic heat; that is, quantities of elements proportional in
mass to their atomic weights require approximately equal increments
of heat to raise their temperature through an equal interval, say 1°.
This is the formulation of Dulong and Petit’s law previously alluded
to. But here we meet with irregularities, which have up till now
defied classification. The heat imparted to an aggregation of atoms
is not expended solely in raising their temperature; other work is
done also, as is generally supposed, in the nature of expansion, or
separation of parts, as in overcoming the attraction between the atoms
in the molecule, or in imparting special motion to the atoms; such
work, however, involves an expenditure of energy which is either very
small in proportion to the total energy imparted as heat, or is nearly
the same for all elements. At present we cannot decide between these
alternatives, owing to the lack of knowledge of the nature of liquids
and solids. The main fact, however, is incontestable: that the heat
energy required to raise different elements through the same interval
of temperature is the same, not for equal masses or weights of the
substances, but for their _atomic_ masses.
Again, many compounds when dissolved in water conduct an electric
current, while they themselves are decomposed; and the different
ingredients of the compound are often deposited at the points where the
electric current enters or leaves the liquid. Where they are not so
deposited, it is usually because of their action on the solvent water.
Now Faraday found that when elements are deposited, equal quantities of
electricity are conveyed either by equal numbers of atoms or by some
simple fraction of these numbers. To this fraction we apply the term
_valency_. Thus we say that an atom of oxygen is bivalent, or a dyad,
because it conveys twice as much electricity through the liquid in
which it is present as an atom of hydrogen, which is termed univalent,
or a monad. Here we see a direct connection between the conveyance of
an electric charge and the atomic weight. The electric unit of quantity
is in fact defined as that which can be conveyed by a certain weight of
hydrogen, by 8 times that weight of oxygen, by 108 times that weight of
silver, and so on. Therefore the electrical unit is connected, not with
unit of mass, or with the gravitational unit, but with the atomic unit.
We have therefore a number of systems, each capable of being equated
to a unit of energy, but of which the terms are in some as yet unknown
way related to each other, and often more directly than they can be
related to mass and weight. This relation is only an approximate one
in the case of specific heats; it appears to be an absolute one in the
conveyance of electrical energy. The arrangement of the elements in
the periodic table must therefore be considered by the light of such
general views.
I would venture to suggest, as a tentative method of solving this
problem, that it be considered whether mass or weight are such
invariable properties of matter as have generally been taken for
granted. That the relative weights or masses in which elements combine
always retain their invariable proportion is true, so far as we can
determine; but it must be remembered that we cannot cause them to
combine except under a very limited series of conditions. For example,
the act of combination equalises the temperature of two combining
atoms; it also, in all probability, equalises their electrical charges.
It is a legitimate speculation whether, could we maintain a difference
in their temperature or in their electric potential, their atomic
weights might not also change. Indeed, we are ignorant whether mass
is changed by alteration of temperature. Experiments made by Sir
John Airy, and interpreted by Professor Hicks, appear to show that
variation of temperature is not without some influence on gravitational
attraction. Others by Professor Landolt point in the same direction.
It therefore appears to me not impossible that the mass of the atoms
may be affected by the various and different properties which they
possess, some to a greater, some to a lesser extent. It must be
admitted that atoms differ from each other in the readiness with
which they combine with those of the same kind to form molecules; and
that molecules of different elements differ from each other in their
capacity to form molecular aggregates. Take, for example, such cases
as caesium and fluorine, each intensely active, but towards different
objects: caesium the most electro-positive of the metals, and fluorine
the most electro-negative of elements. Surely their activity must be
due to some cause which cannot but exert influence on their other
properties, such as their mass and their gravitational attraction, as
it doubtless has influence on their specific heats, and on many of
their other physical properties. And contrast these instances with
helium and with argon, the most indifferent of substances, the atoms
of which are unwilling, and apparently unable, to pair even with
themselves; it is hardly conceivable that these peculiarities should
leave their other, and, as we are in the habit of thinking, invariable,
properties unaffected. I venture to suggest that these powers of
combination, due to some configuration or to some attractive force,
tend to lessen the gravitational attraction by which we measure their
atomic weights; that helium and argon, which possess little, if any,
of such power to combine, show what may be termed the normal atomic
weights, inasmuch as their gravitational attraction is subject to no
deduction attributable to their reacting powers.
I cannot but think that, when some numerical values are assigned to
this combinational power, it will be found that they will so increase
their atomic weights as to display that regularity which, so far as we
can see at present, is conspicuous by its absence.
I am aware that these suggestions are of a wholly speculative
character; and yet I venture to put them forward in the firm conviction
that no true progress in knowledge has ever been made without such
speculations. It was the speculative phlogistic theory which combined
phenomena apparently so distinct as the burning of a candle and the
rusting of iron. It is true that that theory is now a phantom of the
past, yet it served its purpose in directing attention to phenomena
of a similar character. It would be easy to multiply instances of
the kind; in almost every case some useful object has been served by
speculation preceding exact knowledge. The object of science, as indeed
of inquiry in all departments of human interest, is to reconcile the
world of man with the world of nature, and to endeavour to know in part
that of which we hope one day to attain to a perfect knowledge.
THE END
[30] These considerations would hold on the assumption that no
combination takes place between chlorine and bromine.
_Printed by_ R. & R. CLARK, LIMITED, _Edinburgh_.
End of Project Gutenberg's The Gases of the Atmosphere, by William Ramsay