Encyclopaedia Britannica > Volume 21, ROT-Siam

STE-CLAIRE DEYILLE
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of which he discovered the hydro-carbon toluene. But he soon
abandoned organic chemistry, and his most important work vas m
inorganic and thermal chemistry. In 1850 he discovered anhy¬
drous nitric acid, a substance interesting not only m itselt but as
the first obtained of an important group, the so-called anhy¬
drides ” of the monobasic acids. In 1855 lie succeeded m obtaining
aluminium in mass. This metal, of which clay is the hydrated
silicate, is of course one of the most abundant of metals, but was
not obtained in the metallic state until Wohler m 1827 decomposed
its chloride by means of potassium. The aluminium thus prepared
was in the form of a fine powder, and, although the isolation of
the metal was of great theoretical importance, there did not seem
much prospect of a practical application of the discovery. In 1845
Wohler returned to the .subject and by using large quantities of
material obtained small globules of an obviously metallic character.
Deville, who knew only Wohler’s paper of 1827, set to work to
prepare aluminium, not for the sake of the metal itself, but with
the view of procuring by the action of aluminium on chloride of
aluminium a lower chloride from which a series of new compounds
corresponding to the ferrous salts might be obtained. He did not
succeed in this, but he did succeed in producing globules of alumi¬
nium of considerable size. This led him to perfect the process, and
ultimately he devised a method by which aluminium could be pre¬
pared on a large scale. The first use to which he put the metal
was to make a medal with the name of Wohler and the date 1827.
In connexion with the preparation of aluminium may be mentioned
Deville’s investigations, partly with Wohler, into the allotropic
forms of silicon and horon.
Along with Debray, Deville studied the platinum metals ; their
object was on the one hand to prepare the six metals in a state of
purity and on the other to obtain a suitable metal for the standard
metre. In the course of these investigations large quantities of
platinum and of the alloys of platinum and iridium were fused and
cast,1 and the methods used for obtaining the necessary high
temperatures were applied to the fusion of other refractory metals,
such as cobalt, nickel, chromium, and manganese.
Along with Troost, Deville devised a method for determining
the density of vapours at very high temperatures and applied it
to the cases of sulphur, selenium, tellurium, zinc, cadmium, and
many other substances boiling at temperatures up to 1400° C. The
interesting and important results have been already described (see
Chemistry and Molecule). Deville made a large number of
ingenious experiments on the artificial production of minerals.
Among these may be specially mentioned the formation of apatite
and isomorphous minerals and of crystallized oxides. Deville and
Caron found that when the vapour of a metallic fluoride acts on
fused boracic acid the fluorine and the oxygen change places, a
metallic oxide remains in crystals, while the gaseous fluoride of
boron escapes. In this way they prepared corundum (crystallized
oxide of aluminium) and sapphire, ruby and emerald ' coloured
forms of corundum were obtained by mixing small quantities of
fluoride of chromium with the fluoride of aluminium. Another
method discovered by Deville for the preparation of crystallized
oxides is of great interest. When an amorphous oxide—such as
amorphous ferric oxide—is heated to redness and exposed to a slow
current of hydrochloric acid gas, it gradually changes into a crystal¬
line oxide of the same composition. In this way Deville obtained
hsematite, tinstone, periclase, and other crystalline oxides. This
conversion of an amorphous into a crystalline substance without
change of composition, by the action of a gas (in this case hydro¬
chloric acid) which itself undergoes no change, is one of those
mysterious processes which used to be referred to a “ catalytic
force” or called ‘‘actions by contact”; like many such actions,
this has been shown by Deville to belong to the same class of
phenomena as dissociation.
This leads us to Deville’s greatest contribution to general
chemistry. Many chemical actions have been long known which
take place either in the one or the other sense according to certain
conditions. For instance, if a tube containing metallic iron is heated
to redness and steam passed through it, water is decomposed, black
oxide of iron is formed, and hydrogen escapes. If, on the other hand,
the tube is filled with black oxide of iron and hydrogen passed
through, the oxide is reduced and water is formed. Both of these
opposite changes occur at the very same temperature. Again, a
solution of sulph-hydrate of potassium is completely decomposed
by passing a current of carbonic acid gas through it for a sufficient
time, sulphuretted hydrogen being given off and bicarbonate re¬
maining in solution. But exactly the opposite happens if we begin
with bicarbonate and pass sulphuretted hydrogen gas through it:
carbonic acid gas escapes and the solution ultimately contains
nothing but sulph-hydrate. An imperfect, unsatisfactory ex¬
planation of some of the phenomena of which these are examples
was given by Berthollet; it remained for Deville to give a general
theory and show their relation to such physical phenomena as
1 The metre commission fused a quarter of a ton of the alloy at a single
operation.
evaporation and condensation. This he did by his experimental
work on “ Dissociation ” and his theoretical discussion of the facts
in papers published in the Comptes Rendus. He gave a very com¬
plete and clear account of the whole subject in a lecture delivered
before the Chemical Society of Paris in 1866.
As illustrations we shall take a few cases as different from one
another as possible.
It has long been known that carbonate of lime—limestone—
when heated is decomposed into quicklime and carbonic acid gas,
and that this decomposition takes place the more quickly the more
thoroughly the carbonic acid produced is removed. Sir James Hall
showed that, if the carbonate of lime is heated in a closed vessel
strong enough to resist the pressure of the carbonic acid gas, it can
be fused, only a small part undergoing decomposition. Deville
examined this relation quantitatively and showed that, if in a closed
vessel we have quicklime, carbonate of lime, and carbonic acid gas,
the pressure of the carbonic acid gas depends on the temperature
only, and is quite independent of the quantity of the quicklime or
of the carbonate of lime, as long as there is some, however little,
of both, and is also quite uninfluenced by the presence of other
gases. It will be seen that this case exactly resembles that of the
evaporation of water. In a closed vessel containing liquid water
and water-vapour the pressure of the water-vapour tlepends on the
temperature only and is independent of the quantity of liquid water,
as long as there is any, and is not influenced by the presence of
other gases. In both cases, if we disturb the equilibrium and then
leave things to themselves the equilibrium is restored. If in the
first case we diminish the pressure of the carbonic acid gas, some
carbonate of lime decomposes, yielding carbonic acid gas until the
pressure is raised to what it was ; if we increase the pressure, some
of the carbonic acid combines with quicklime until the pressure is
reduced to what it was before. In the second case, if we diminish
the pressure, some of the liquid water evaporates ; if we increase it,
some of the water-vapour condenses, and so the pressure is restored.
Kise of temperature causes in the one case evaporation of water, in
the other decomposition of carbonate of lime,—in both increase of
pressure. Lowering of temperature causes in the one case condensa¬
tion of water-vapour, in the other combination of quicklime and
carbonic acid gas,—in both diminution of pressure.
As a second instance we may take the dissociation of water. Just
as water-vapour condenses into liquid water under certain condi¬
tions, but always with the evolution of heat (latent heat of vapour),
so the mixture of oxygen and hydrogen in the proper proportion to
form water combines, under certain conditions, to form water-vapour,
but always with the evolution of heat (heat of combination). In
both cases we have change of state but no change of composition,
and in both we have evolution of heat. In the first case we can
reverse the process: heat the liquid water, heat becomes latent,
liquid water changes into water-vapour. There is a certain definite
pressure of water-vapour corresponding to the temperature: raise
the temperature, more water evaporates, the pressure of water-
vapour increases. It occurred to Deville, to whom both changes
were equally physical, that in the second case the process should
be reversible also,—that on heating the water-vapour it ought to
decompose into oxygen and hydrogen, heat disappearing here also,
and that, as there is a definite pressure of water-vapour correspond¬
ing to the temperature (often called the tension of water-vapour), so
there should be a definite ratio of the pressure of hydrogen and
oxygen to that of water-vapour (the tension of dissociation). Deville
showed in the most conclusive manner that this is the case and
devised ingenious arrangements for proving the actual occurrence
of dissociation.
Another case very fully investigated by Deville is that already
mentioned,—viz., the action of water-vapour on iron, and of
hydrogen on oxide of iron. He showed that, for a fixed temper¬
ature, water-vapour and hydrogen are in equilibrium in presence of
iron and oxide of iron when the pressures of the two gases, hydrogen
and water-vapour, are in a certain ratio quite independent of the
quantity of the iron or of the oxide of iron, as long as there is some
of each. If the ratio is changed, say by increasing the pressure of
the water-vapour, chemical action takes place: water is decomposed,
oxide of iron is formed, and hydrogen set free. Again, if the pressure
of the vTater-vapour is diminished, part of the hydrogen acts on oxide
of iron, reducing it and forming water. In both cases the ratio of
pressures is restored. This gives an easy explanation of the appa¬
rently anomalous results mentioned above. When a current of
hydrogen is passed over oxide of iron the water-vapour produced
is swept away as fast as it is formed ; the ratio of the pressure ot
hydrogen to that of water-vapour is therefore always greater than
that required for equilibrium and reduction of iron, ami formation
of water goes on continuously until all the oxide of iron is reduced.
In the same way, a current of water-vapour carries away the hydrogen
as fast as it is produced ; the ratio of the pressure of hydrogen to
that of water-vapour is always less than that required for equili¬
brium, and the oxidation of iron and production of hydrogen goes
on until no metallic iron remains. Exactly the same explanation
applies to the action of carbonic acid gas on solution of sulph-