Chapter II: The Composition of Water, Hydrogen (3)

Hydrogen is vigorously condensed by certain solids; for example, by charcoal and by spongy platinum. If a piece of freshly ignited charcoal be introduced into a cylinder full of hydrogen standing in a mercury bath, then the charcoal absorbs as much as twice its volume of hydrogen. Spongy platinum condenses still more hydrogen. But _palladium_, a grey metal which occurs with platinum, absorbs more hydrogen than any other metal. Graham showed that when heated to a red heat and cooled in an atmosphere of hydrogen, palladium retains as much as 600 volumes of hydrogen. When once absorbed it retains the hydrogen at the ordinary temperature, and only parts with it when heated to a red heat.[36] This capacity of certain dense metals for the absorption of hydrogen explains the property of hydrogen of passing through metallic tubes.[37] It is termed _occlusion_, and presents a similar phenomenon to solution; it is based on the capacity of metals of forming unstable easily dissociating compounds[38] with hydrogen, similar to those which salts form with water.

[36] The property of palladium of absorbing hydrogen, and of increasing
in volume in so doing, may be easily demonstrated by taking a
sheet of palladium varnished on one side, and using it as a
cathode. The hydrogen which is evolved by the action of the
current is retained by the unvarnished surface, as a consequence
of which the sheet curls up. By attaching a pointer (for
instance, a quill) to the end of the sheet this bending effect is
rendered strikingly evident, and on reversing the current (when
oxygen will be evolved and combine with the absorbed hydrogen,
forming water) it may be shown that on losing the hydrogen the
palladium regains its original form.

[37] Deville discovered that iron and platinum become pervious to
hydrogen at a red heat. He speaks of this in the following
terms:--'The permeability of such homogeneous substances as
platinum and iron is quite different from the passage of gases
through such non-compact substances as clay and graphite. The
permeability of metals depends on their expansion, brought
about by heat, and proves that metals and alloys have a certain
porosity.' However, Graham proved that it is only hydrogen which
is capable of passing through the above-named metals in this
manner. Oxygen, nitrogen, ammonia, and many other gases, only
pass through in extremely minute quantities. Graham showed that
at a red heat about 500 c.c. of hydrogen pass per minute through
a surface of one square metre of platinum 1·1 mm. thick, but that
with other gases the amount transmitted is hardly perceptible.
Indiarubber has the same capacity for allowing the transference
of hydrogen through its substance (_see_ Chapter III.), but at
the ordinary temperature one square metre, 0·014 mm. thick,
transmits only 127 c.c. of hydrogen per minute. In the experiment
on the decomposition of water by heat in porous tubes, the clay
tube may be exchanged for a platinum one with advantage. Graham
showed that by placing a platinum tube containing hydrogen under
these conditions, and surrounding it by a tube containing air,
the transference of the hydrogen may be observed by the decrease
of pressure in the platinum tube. In one hour almost all the
hydrogen (97 p.c.) had passed from the tube, without being
replaced by air. It is evident that the occlusion and passage
of hydrogen through metals capable of occluding it are not only
intimately connected together, but are dependent on the capacity
of metals to form compounds of various degrees of stability with
hydrogen--like salts with water.

[38] It appeared on further investigation that palladium gives a
definite compound, Pd_{2}H (_see_ further) with hydrogen; but
what was most instructive was the investigation of sodium
hydride, Na_{2}H, which clearly showed that the origin and
properties of such compounds are in entire accordance with the
conceptions of dissociation.

Since hydrogen is a gas which is difficult to condense, it is
little soluble in water and other liquids. At 0° a hundred
volumes of water dissolve 1·9 volume of hydrogen, and alcohol 6·9
volumes measured at 0° and 760 mm. Molten iron absorbs hydrogen,
but in solidifying, it expels it. The solution of hydrogen by
metals is to a certain degree based on its affinity for metals,
and must be likened to the solution of metals in mercury and to
the formation of alloys. In its chemical properties hydrogen,
as we shall see later, has much of a metallic character. Pictet
(_see_ Note 31) even affirms that liquid hydrogen has metallic
properties. The metallic properties of hydrogen are also evinced
in the fact that it is a good conductor of heat, which is not the
case with other gases (Magnus).

At the ordinary temperature hydrogen very feebly and rarely enters into chemical reaction. The capacity of gaseous hydrogen for reaction becomes evident only under a change of circumstances--by compression, heating, or the action of light, or at the moment of its evolution. However, under these circumstances it _combines_ directly with only a very few of the elements. Hydrogen combines directly with oxygen, sulphur, carbon, potassium, and certain other elements, but it does not combine directly with either the majority of the metals or with nitrogen, phosphorus, &c. Compounds of hydrogen with certain elements on which it does not act directly are, however, known; they are not obtained by a direct method, but by reactions of decomposition, or of double decomposition, of other hydrogen compounds. The property of hydrogen of combining with oxygen at a red heat determines its combustibility. We have already seen that hydrogen easily takes fire, and that it then burns with a pale--that is, non-luminous--flame.[39] Hydrogen does not combine with the oxygen of the atmosphere at the ordinary temperature; but this combination takes place at a red heat,[40] and is accompanied by the evolution of much heat. The product of this combination is water--that is, a compound of oxygen and hydrogen. This is the _synthesis of water_, and we have already noticed its analysis or decomposition into its component parts. The synthesis of water may be very easily observed if a cold glass bell jar be placed over a burning hydrogen flame, and, better still, if the hydrogen flame be lighted in the tube of a condenser. The water will condense in drops as it is formed on the walls of the condenser and trickle down.[41]

[39] If it be desired to obtain a perfectly colourless hydrogen
flame, it must issue from a platinum nozzle, as the glass end of
a gas-conducting tube imparts a yellow tint to the flame, owing
to the presence of sodium in the glass.

[40] Let us imagine that a stream of hydrogen passes along a tube, and
let us mentally divide this stream into several parts,
consecutively passing out from the orifice of the tube. The first
part is lighted--that is, brought to a state of incandescence,
in which state it combines with the oxygen of the atmosphere. A
considerable amount of heat is evolved in the combination. The
heat evolved then, so to say, ignites the second part of hydrogen
coming from the tube, and, therefore, when once ignited, the
hydrogen continues to burn, if there be a continual supply of it,
and if the atmosphere in which it burns be unlimited and contains
oxygen.

[41] The combustibility of hydrogen may be shown by the direct
decomposition of water by sodium. If a pellet of sodium be thrown
into a vessel containing water, it floats on the water and
evolves hydrogen, which may be lighted. The presence of sodium
imparts a yellow tint to the flame. If potassium be taken, the
hydrogen bursts into flame spontaneously, because sufficient heat
is evolved in the reaction to ignite the hydrogen. The flame is
coloured violet by the potassium. If sodium be thrown not on
to water, but on to an acid, it will evolve more heat, and the
hydrogen will then also burst into flame. These experiments must
be carried on with caution, as, sometimes towards the end, a
mass of sodium oxide (Note 8) is produced, and flies about; it
is therefore best to cover the vessel in which the experiment is
carried on.

Light does not aid the combination of hydrogen and oxygen, so that a mixture of these two gases does not change when exposed to the action of light; but an electric spark acts just like a flame, and this is taken advantage of for inflaming a mixture of oxygen and hydrogen, or detonating gas, inside a vessel, as will be explained in the following chapters. As hydrogen (and oxygen also) is condensed by spongy platinum, by which a rise of temperature ensues, and as platinum acts by contact (Introduction), therefore hydrogen also combines with oxygen, under the influence of platinum, as Döbereiner showed. If spongy platinum be thrown into a mixture of hydrogen and oxygen, an explosion takes place. If a mixture of the gases be passed over spongy platinum, combination also ensues, and the platinum becomes red-hot.[42]

[42] This property of spongy platinum is made use of in the so-called
hydrogen cigar-lighter. It consists of a glass cylinder or
beaker, inside which there is a small lead stand (which is not
acted on by sulphuric acid), on which a piece of zinc is laid.
This zinc is covered by a bell, which is open at the bottom
and furnished with a cock at the top. Sulphuric acid is poured
into the space between the bell and the sides of the outer
glass cylinder, and will thus compress the gas in the bell. If
the cock of the cylinder be opened the gas will escape by it,
and will be replaced by the acid, which, coming into contact
with the zinc, evolves hydrogen, and it will escape through the
cock. If the cock be closed, then the hydrogen evolved will
increase the pressure of the gas in the bell, and thus again
force the acid into the space between the bell and the walls of
the outer cylinder. Thus the action of the acid on the zinc may
be stopped or started at will by opening or shutting the cock,
and consequently a stream of hydrogen may be always turned on.
Now, if a piece of spongy platinum be placed in this stream, the
hydrogen will take light, because the spongy platinum becomes
hot in condensing the hydrogen and inflames it. The considerable
rise in temperature of the platinum depends, among other things,
on the fact that the hydrogen condensed in its pores comes into
contact with previously absorbed and condensed atmospheric
oxygen, with which hydrogen combines with great facility in this
form. In this manner the hydrogen cigar-lighter gives a stream of
burning hydrogen when the cock is open. In order that it should
work regularly it is necessary that the spongy platinum should be
quite clean, and it is best enveloped in a thin sheet of platinum
foil, which protects it from dust. In any case, after some time
it will be necessary to clean the platinum, which may be easily
done by boiling it in nitric acid, which does not dissolve the
platinum, but clears it of all dirt. This imperfection has given
rise to several other forms, in which an electric spark is made
to pass before the orifice from which the hydrogen escapes. This
is arranged in such a manner that the zinc of a galvanic element
is immersed when the cock is turned, or a small coil giving a
spark is put into circuit on turning the hydrogen on.

Although gaseous hydrogen does not act directly[43] on many substances, yet in a _nascent state_ reaction often takes place. Thus, for instance, water on which sodium amalgam is acting contains hydrogen in a nascent state. The hydrogen is here evolved from a liquid, and at the first moment of its formation must be in a condensed state.[44] In this condition it is capable of reacting on substances on which it does not act in a gaseous state.[44 bis] Reactions of substitution or displacement of metals by hydrogen at the moment of its formation are particularly numerous.[45]

[43] Under conditions similar to those in which hydrogen combines
with oxygen it is also capable of combining with chlorine. A
mixture of hydrogen and chlorine explodes on the passage of an
electric spark through it, or on contact with an incandescent
substance, and also in the presence of spongy platinum; but,
besides this, the action of light alone is enough to bring about
the combination of hydrogen and chlorine. If a mixture of equal
volumes of hydrogen and chlorine be exposed to the action of
sunlight, complete combination rapidly ensues, accompanied by a
report. Hydrogen does not combine directly with carbon, either at
the ordinary temperature or by the action of heat and pressure.
But if an electric current be passed through carbon electrodes
at a short distance from each other (as in the electric light
or voltaic arc), so as to form an electric arc in which the
particles of carbon are carried from one pole to the other, then,
in the intense heat to which the carbon is subjected in this
case, it is capable of combining with hydrogen. A gas of peculiar
smell called acetylene, C_{2}H_{2}, is thus formed from carbon
and hydrogen.

[44] There is another explanation of the facility with which hydrogen
reacts in a nascent state. We shall afterwards learn that the
molecule of hydrogen contains two atoms, H_{2}, but there are
elements the molecules of which only contain one atom--for
instance, mercury. Therefore, every reaction of gaseous hydrogen
must be accompanied by the disruption of that bond which exists
between the atoms forming a molecule. At the moment of evolution,
however, it is supposed that free atoms exist, and in this
condition, according to the hypothesis, act energetically. This
hypothesis is not based upon facts, and the idea that hydrogen is
condensed at the moment of its evolution is more natural, and is
in accordance with the fact (Note 12) that compressed hydrogen
displaces palladium and silver (Brunner, Beketoff)--that is, acts
as at the moment of its liberation.

[44 bis] There is a very intimate and evident relation between the
phenomena which take place in the action of spongy platinum and
the phenomena of the action in a nascent state. The combination
of hydrogen with aldehyde may be taken as an example. Aldehyde is
a volatile liquid with an aromatic smell, boiling at 21°, soluble
in water, and absorbing oxygen from the atmosphere, and in this
absorption forming acetic acid--the substance which is found in
ordinary vinegar. If sodium amalgam be thrown into an aqueous
solution of aldehyde, the greater part of the hydrogen evolved
combines with the aldehyde, forming alcohol--a substance also
soluble in water, which forms the principle of all spirituous
liquors, boils at 78°, and contains the same amount of oxygen
and carbon as aldehyde, but more hydrogen. The composition of
aldehyde is C_{2}H_{4}O, that of alcohol C_{2}H_{6}O.

[45] When, for instance, an acid and zinc are added to a salt of
silver, the silver is reduced; but this may be explained as a
reaction of the zinc, and not of the hydrogen at the moment
of its formation. There are, however, examples to which this
explanation is entirely inapplicable; thus, for instance,
hydrogen, at the moment of its liberation easily takes up oxygen
from its compounds with nitrogen if they be in solution, and
converts the nitrogen into its hydrogen-compound. Here the
nitrogen and hydrogen, so to speak, meet at the moment of their
liberation, and in this state combine together.

It is evident from this that the elastic gaseous state of
hydrogen fixes the limit of its energy: prevents it from entering
into those combinations of which it is capable. In the nascent
state we have hydrogen which is not in a gaseous state, and its
action is then much more energetic. At the moment of evolution
that heat, which would be latent in the gaseous hydrogen, is
transmitted to its molecules, and consequently they are in a
state of strain, and can hence act on many substances.

Metals, as we shall afterwards see, are in many cases able to replace each other; they also, and in some cases still more easily, replace and are replaced by hydrogen. We have already seen examples of this in the formation of hydrogen from water, sulphuric acid, &c. In all these cases the metals sodium, iron, or zinc displace the hydrogen which occurs in these compounds. Hydrogen may be displaced from many of its compounds by metals in exactly the same manner as it is displaced from water; so, for example, hydrochloric acid, which is formed directly by the combination of hydrogen with chlorine, gives hydrogen by the action of a great many metals, just as sulphuric acid does. Potassium and sodium also displace hydrogen from its compounds with nitrogen; it is only from its compounds with carbon that hydrogen is not displaced by metals. Hydrogen, in its turn, is able to replace metals; this is accomplished most easily on heating, and with those metals which do not themselves displace hydrogen. If hydrogen be passed over the compounds of many metals with oxygen at a red heat, it takes up the oxygen from the metals and displaces them just as it is itself displaced by metals. If hydrogen be passed over the compound of oxygen with copper at a red heat, then metallic copper and water are obtained--CuO + H_{2} = H_{2}O + Cu. This kind of double decomposition is called _reduction_ with respect to the metal, which is thus reduced to a metallic state from its combination with oxygen. But it must be recollected that all metals do not displace hydrogen from its compound with oxygen, and, conversely, hydrogen is not able to displace all metals from their compounds with oxygen; thus it does not displace potassium, calcium, or aluminium from its compounds with oxygen. If the metals be arranged in the following series: K, Na, Ca, Al ... Fe, Zn, Hg ... Cu, Pb, Ag, Au, then the first are able to take up oxygen from water--that is, displace hydrogen--whilst the last do not act thus, but are, on the contrary, reduced by hydrogen--that is, have, as is said, a less affinity for oxygen than hydrogen, whilst potassium, sodium, and calcium have more. This is also expressed by the amount of heat evolved in the act of combination with oxygen (_see_ Note 7), and is shown by the fact that potassium and sodium and other similar metals evolve heat in decomposing water; but copper, silver, and the like do not do this, because in combining with oxygen they evolve less heat than hydrogen does, and therefore it happens that when hydrogen reduces these metals heat is evolved. Thus, for example, if 16 grams of oxygen combine with copper, 38,000 units of heat are evolved; and when 16 grams of oxygen combine with hydrogen, forming water, 69,000 units of heat are evolved; whilst 23 grams of sodium, in combining with 16 grams of oxygen, evolve 100,000 units of heat. This example clearly shows that chemical reactions which proceed directly and unaided evolve heat. Sodium decomposes water and hydrogen reduces copper, because they are _exothermal_ reactions, or those which evolve heat; copper does not decompose water, because such a reaction would be accompanied by an absorption (or secretion) of heat, or belongs to the class of _endothermal_ reactions in which heat is absorbed; and such reactions do not generally proceed directly, although they may take place with the aid of energy (electrical, thermal, &c.) borrowed from some foreign source.[46]

[46] Several numerical data and reflections bearing on this matter are
enumerated in Notes 7, 9, and 11. It must be observed that the
action of iron or zinc on water is reversible. But the reaction
CuO + H_{2} = Cu + H_{2}O is not reversible; the difference
between the degrees of affinity is very great in this case,
and, therefore, so far as is at present known, no hydrogen is
liberated even in the presence of a large excess of water. It
is to be further remarked, that under the conditions of the
dissociation of water, copper is not oxidised by water, because
the oxide of copper is reduced by free hydrogen. If a definite
amount of a metal and acid be taken and their reaction be carried
on in a closed space, then the evolution of hydrogen will cease,
when its tension equals that at which compressed hydrogen
displaces the metal. The result depends upon the nature of the
metal and the strength of the solution of acid. Tammann and
Nernst (1892) found that the metals stand in the following order
in respect to this limiting tension of hydrogen:--Na, Mg, Zn, Al,
Cd, Fe, Ni.

The reduction of metals by hydrogen is taken advantage of for _determining the exact composition of water by weight_. Copper oxide is usually chosen for this purpose. It is heated to redness in hydrogen, and the quantity of water thus formed is determined, when the quantity of oxygen which occurs in it is found from the loss of weight of the copper oxide. The copper oxide must be weighed immediately before and after the experiment. The difference shows the weight of the oxygen which entered into the composition of the water formed. In this manner only solids have to be weighed, which is a very great gain in the accuracy of the results obtained.[47] Dulong and Berzelius (1819) were the first to determine the composition of water by this method, and they found that water contains 88·91 of oxygen and 11·09 of hydrogen in 100 parts by weight, or 8·008 parts of oxygen per one part of hydrogen. Dumas (1842) improved on this method,[48] and found that water contains 12·575 parts of hydrogen per 100 parts of oxygen--that is, 7·990 parts of oxygen per 1 part of hydrogen--and therefore it is usually accepted that _water contains eight parts by weight of oxygen to_ one _part by weight of hydrogen_. By whatever method water be obtained, it will always present the same composition. Whether it be taken from nature and purified, or whether it be obtained from hydrogen by oxidation, or whether it be separated from any of its compounds, or obtained by some double decomposition--it will in every case contain one part by weight of hydrogen and eight parts of oxygen. This is because water is a definite chemical compound. Detonating gas, from which it may be formed, is a simple mixture of oxygen and hydrogen, although a mixture of the same composition as water. All the properties of both constituent gases are preserved in detonating gas. Either one or the other gas may be added to it without destroying its homogeneity. The fundamental properties of oxygen and hydrogen are not found in water, and neither of the gases can be directly combined with it. But they may be evolved from it. In the formation of water there is an evolution of heat; for the decomposition of water heat is required. All this is expressed by the words, _Water is a definite chemical compound of hydrogen with oxygen_. Taking the symbol of hydrogen, H, as expressing a unit quantity by weight of this substance, and expressing 16 parts by weight of oxygen by O, we can formulate all the above statements by the chemical symbol of water, H_{2}O. As only definite chemical compounds are denoted by formulæ, having denoted the formula of a compound substance we express by it the entire series of properties which go to make up our conception of a definite compound, and at the same time the quantitative composition of the substance by weight. Further, as we shall afterwards see, formulæ express the volume of the gases contained in a substance. Thus the formula of water shows that it contains two volumes of hydrogen and one volume of oxygen. Besides which, we shall learn that the formula expresses the density of the vapour of a compound, and on this many properties of substances depend, and, as we shall learn, determine the quantities of the bodies entering into reactions. This vapour density we shall find also determines the quantity of a substance entering into a reaction. Thus the letters H_{2}O tell the chemist the entire history of the substance. This is an international language, which endows chemistry with a simplicity, clearness, stability, and trustworthiness founded on the investigation of the laws of nature.

[47] This determination may be carried on in an apparatus like that
mentioned in Note 13 of Chapter I.

[48] We will proceed to describe Dumas' method and results. For this
determination pure and dry copper oxide is necessary. Dumas took
a sufficient quantity of copper oxide for the formation of 50
grams of water in each determination. As the oxide of copper
was weighed before and after the experiment, and as the amount
of oxygen contained in water was determined by the difference
between these weights, it was essential that no other substance
besides the oxygen forming the water should be evolved from
the oxide of copper during its ignition in hydrogen. It was
necessary, also, that the hydrogen should be perfectly pure,
and free not only from traces of moisture, but from any other
impurities which might dissolve in the water or combine with the
copper and form some other compound with it. The bulb containing
the oxide of copper (fig. 26), which was heated to redness,
should be quite free from air, as otherwise the oxygen in the
air might, in combining with the hydrogen passing through the
vessel, form water in addition to that formed by the oxygen
of the oxide of copper. The water formed should be entirely
absorbed in order to accurately determine its quantity. The
hydrogen was evolved in the three-necked bottle. The sulphuric
acid, for acting on the zinc, is poured through funnels into
the middle neck. The hydrogen evolved in the Woulfe's bottle
passes through [U] tubes, in which it is purified, to the bulb,
where it comes into contact with the copper oxide, forms water,
and reduces the oxide to metallic copper; the water formed is
condensed in the second bulb, and any passing off is absorbed in
the second set of [U] tubes. This is the general arrangement of
the apparatus. The bulb with the copper oxide is weighed before
and after the experiment. The loss in weight shows the quantity
of oxygen which entered into the composition of the water formed,
the weight of the latter being shown by the gain in weight of
the absorbing apparatus. Knowing the amount of oxygen in the
water formed, we also know the quantity of hydrogen contained in
it, and consequently we determine the composition of water by
weight. This is the essence of the determination. We will now
turn to certain particulars. In one neck of the three-necked
bottle a tube is placed dipping under mercury. This serves as
a safety-valve to prevent the pressure inside the apparatus
becoming too great from the rapid evolution of hydrogen. If the
pressure rose to any considerable extent, the current of gases
and vapours would be very rapid, and, as a consequence, the
hydrogen would not be perfectly purified, or the water entirely
absorbed in the tubes placed for this purpose. In the third neck
of the Woulfe's bottle is a tube conducting the hydrogen to the
purifying apparatus, consisting of eight [U] tubes, destined
for the purification and testing of the hydrogen. The hydrogen,
evolved by zinc and sulphuric acid, is purified by passing it
first through a tube full of pieces of glass moistened with a
solution of lead nitrate next through silver sulphate; the lead
nitrate retains sulphurette hydrogen, and arseniuretted hydrogen
is retained by the tube with silver sulphate. Caustic potash in
the next [U] tube retains any acid which might come over. The
two following tubes are filled with lumps of dry caustic potash
in order to absorb any carbonic anhydride and moisture which the
hydrogen might contain. The next two tubes, to remove the last
traces of moisture, are filled with phosphoric anhydride, mixed
with lumps of pumice-stone. They are immersed in a freezing
mixture. The small [U] tube contains hygroscopic substances,
and is weighed before the experiment: this is in order to know
whether the hydrogen passing through still retains any moisture.
If it does not, then the weight of this tube will not vary during
the whole experiment, but if the hydrogen evolved still retains
moisture, the tube will increase in weight. The copper oxide is
placed in the bulb, which, previous to the experiment, is dried
with the copper oxide for a long period of time. The air is then
exhausted from it, in order to weigh the oxide of copper in a
vacuum and to avoid the need of a correction for weighing in
air. The bulb is made of infusible glass, that it may be able to
withstand a lengthy (20 hours) exposure to a red heat without
changing in form. The weighed bulb is only connected with the
purifying apparatus after the hydrogen has passed through for
a long time, and after experiment has shown that the hydrogen
passing from the purifying apparatus is pure and does not
contain any air. On passing from the condensing bulb the gas and
vapour enter into an apparatus for absorbing the last traces of
moisture. The first [U] tube contains pieces of ignited potash,
the second and third tubes phosphoric anhydride or pumice-stone
moistened with sulphuric acid. The last of the two is employed
for determining whether all the moisture is absorbed, and is
therefore weighed separately. The final tube only serves as a
safety-tube for the whole apparatus, in order that the external
moisture should not penetrate into it. The glass cylinder
contains sulphuric acid, through which the excess of hydrogen
passes; it enables the rate at which the hydrogen is evolved
to be judged, and whether its amount should be decreased or
increased.

When the apparatus is fitted up it must be seen that all its
parts are hermetically tight before commencing the experiment.
When the previously weighed parts are connected together and the
whole apparatus put into communication, then the bulb containing
the copper oxide is heated with a spirit lamp (reduction does
not take place without the aid of heat), and the reduction of
the copper oxide then takes place, and water is formed. When
nearly all the copper oxide is reduced the lamp is removed and
the apparatus allowed to cool, the current of hydrogen being
kept up all the time. When cool, the drawn-out end of the bulb
is fused up, and the hydrogen remaining in it is exhausted, in
order that the copper may be again weighed in a vacuum. The
absorbing apparatus remains full of hydrogen, and would therefore
present a less weight than if it were full of air, as it was
before the experiment, and for this reason, having disconnected
the copper oxide bulb, a current of dry air is passed through it
until the gas passing from the glass cylinder is quite free from
hydrogen. The condensing bulb and the two tubes next to it are
then weighed, in order to determine the quantity of water formed.
Dumas repeated this experiment many times. The average result was
that water contains 1253·3 parts of hydrogen per 10,000 parts
of oxygen. Making a correction for the amount of air contained
in the sulphuric acid employed for producing the hydrogen,
Dumas obtained the average figure 1251·5, between the extremes
1247·2 and 1256·2. This proves that per 1 part of hydrogen water
contains 7·9904 parts of oxygen, with a possible error of not
more than 1/250, or 0·03, in the amount of oxygen per 1 part of
hydrogen.

Erdmann and Marchand, in eight determinations, found that per
10,000 parts of oxygen water contains an average of 1,252 parts
of hydrogen, with a difference of from 1,258·5 to 1,248·7; hence
per 1 part of hydrogen there would be 7·9952 of oxygen, with an
error of at least 0·05.

Keiser (1888), in America by employing palladium hydride, and
by introducing various fresh precautions for obtaining accurate
results, found the composition of water to be 15·95 parts of
oxygen per 2 of hydrogen.

Certain of the latest determinations of the composition of
water, as also those made by Dumas, always give less than 8, and
on the average 7·98, of oxygen per 1 part of hydrogen. However,
not one of these figures is to be entirely depended on, and for
ordinary accuracy it may be considered that O = 16 when H = 1.