NITROGEN AND AIR
Gaseous _nitrogen_ forms about four-fifths (by volume) of the
atmosphere; consequently the air contains an exceedingly large
mass of it. Whilst entering in so considerable a quantity into the
composition of air, nitrogen does not seem to play any active part in
the atmosphere, the chemical action of which is mainly dependent on the
oxygen it contains. But this is not an entirely correct idea, because
animal life cannot exist in pure oxygen, in which animals pass into an
abnormal state and die; and the nitrogen of the air, although slowly,
forms diverse compounds, many of which play a most important part in
nature, especially in the life of organisms. However, neither plants[1]
nor animals directly absorb the nitrogen of the air, but take it up from
already prepared nitrogenous compounds; further, plants are nourished by
the nitrogenous substances contained in the soil and water, and animals
by the nitrogenous substances contained in plants and in other animals.
Atmospheric electricity is capable of aiding the passage of gaseous
nitrogen into nitrogenous compounds, as we shall afterwards see, and
the resultant substances are carried to the soil by rain, where they
serve for the nourishment of plants. Plentiful harvests, fine crops of
hay, vigorous growth of trees--other conditions being equal--are only
obtained when the soil contains _ready prepared nitrogenous compounds_,
consisting either of those which occur in air and water, or of the
residues of the decomposition of other plants or animals (as in manure).
The nitrogenous substances contained in animals have their origin in
those substances which are formed in plants. Thus the nitrogen of the
atmosphere is the origin of all the nitrogenous substances occurring in
animals and plants, although not directly so, but after first combining
with the other elements of air.
[1] See Note 15 bis.
The nitrogenous compounds which enter into the composition of plants and
animals are of primary importance; no vegetable or animal cell--that
is, the elementary form of organism--exists without containing a
nitrogenous substance, and moreover organic life manifests itself
primarily in these nitrogenous substances. The germs, seeds, and
those parts by which cells multiply themselves abound in nitrogenous
substances; the sum total of the phenomena which are proper to organisms
depend primarily on the chemical properties of the nitrogenous
substances which enter into their composition. It will be sufficient,
for instance, to point out the fact that vegetable and animal organisms,
clearly distinguishable as such, are characterised by a different degree
of energy in their nature, and at the same time by a difference in the
amount of nitrogenous substances they contain. In plants, which compared
with animals possess but little activity, being incapable of independent
movement, &c., the amount of nitrogen is very much less than in animals,
whose tissues are almost exclusively formed of nitrogenous substances.
It is remarkable that the nitrogenous parts of plants, chiefly of the
lower orders, sometimes present both forms and properties which approach
to those of animal organisms; for example, the zoospores of sea-weeds,
or those parts by means of which the latter multiply themselves. These
zoospores on leaving the sea-weed in many respects resemble the lower
orders of animal life, having, like the latter, the property of moving.
They also approach the animal kingdom in their composition, their outer
coating containing nitrogenous matter. Directly the zoospore becomes
covered with that non-nitrogenous or cellular coating which is proper to
all the ordinary cells of plants, it loses all resemblance to an animal
organism and becomes a small plant. It may be thought from this that the
cause of the difference in the vital processes of animals and plants
is the different amount of nitrogenous substances they contain. The
nitrogenous substances which occur in plants and animals appertain to a
series of exceedingly complex and very changeable chemical compounds;
their elementary composition alone shows this; besides nitrogen, they
contain carbon, hydrogen, oxygen, and sulphur. Being distinguished by a
very great instability under many conditions in which other compounds
remain unchanged, these substances are fitted for those perpetual
changes which form the first condition of vital activity. These complex
and changeable nitrogenous substances of the organism are called
_proteïd substances_. The white of eggs is a familiar example of such a
substance. They are also contained in the flesh of animals, the curdy
elements of milk, the glutinous matter of wheaten flour, or so-called
gluten, which forms the chief component of macaroni, &c.
Nitrogen occurs in the earth's crust, in compounds either forming the
remains of plants and animals, or derived from the nitrogen of the
atmosphere as a consequence of its combination with the other component
parts of the air. It is not found in other forms in the earth's crust;
so that nitrogen must be considered, in contradistinction to oxygen,
as an element which is purely superficial, and does not extend to the
depths of the earth.[1 bis]
[1 bis] The reason why there are no other nitrogenous substances within
the earth's mass beyond those which have come there with the
remains of organisms, and from the air with rain-water, must be
looked for in two circumstances. In the first place, in the
instability of many nitrogenous compounds, which are liable to
break up with the formation of gaseous nitrogen; and in the second
place in the fact that the salts of nitric acid, forming the
product of the action of air on many nitrogenous and especially
organic compounds, are very soluble in water, and on penetrating
into the depths of the earth (with water) give up their oxygen. The
result of the changes of the nitrogenous organic substances which
fall into the earth is without doubt frequently, if not invariably,
the formation of gaseous nitrogen. Thus the gas evolved from coal
always contains much nitrogen (together with marsh gas, carbonic
anhydride, and other gases).
_Nitrogen is liberated_ in a free state in the decomposition of the
_nitrogenous organic substances_ entering into the composition of
organisms--for instance, on their combustion. All organic substances
burn when heated to redness with oxygen (or substances readily yielding
it, such as oxide of copper); the oxygen combines with the carbon,
sulphur, and hydrogen, and the nitrogen is evolved in a free state,
because at a high temperature it does not form any stable compound,
but remains uncombined. Carbonic anhydride and water are formed from
the carbon and hydrogen respectively, and therefore to obtain pure
nitrogen it is necessary to remove the carbonic anhydride from the
gaseous products obtained. This may be done very easily by the action of
alkalis--for instance, caustic soda. The amount of nitrogen in organic
substances is determined by a method founded on this.
It is also very easy to obtain _nitrogen from air_, because oxygen
combines with many substances. Either phosphorus or metallic copper is
usually employed for removing the oxygen from air, but, naturally, a
number of other substances may also be used. If a small saucer on which
a piece of phosphorus is laid be placed on a cork floating on water, and
the phosphorus be lighted, and the whole covered with a glass bell jar,
then the air under the jar will be deprived of its oxygen, and nitrogen
only will remain, owing to which, on cooling, the water will rise to
a certain extent in the bell jar. The same object (procuring nitrogen
from air) is attained much more conveniently and perfectly by passing
air through a red-hot tube containing copper filings. At a red heat,
metallic copper combines with oxygen and gives a black powder of copper
oxide. If the layer of copper be sufficiently long and the current of
air slow, all the oxygen will be absorbed, and nitrogen alone will pass
from the tube.[2]
[2] Copper (best as turnings, which present a large surface) absorbs
oxygen, forming CuO, at the ordinary temperature in the presence
of solutions of acids, or, better still, in the presence of a
solution of ammonia, when it forms a bluish-violet solution of
oxide of copper in ammonia. Nitrogen is very easily procured by
this method. A flask filled with copper turnings is closed with a
cork furnished with a funnel and stopcock. A solution of ammonia
is poured into the funnel, and caused to drop slowly upon the
copper. If at the same time a current of air be slowly passed
through the flask (from a gasholder), then all the oxygen will
be absorbed from it and the nitrogen will pass from the flask.
It should be washed with water to retain any ammonia that may be
carried off with it.
Nitrogen may also be procured from many of its _compounds with oxygen[3]
and hydrogen_,[4] but the best fitted for this purpose is a saline
mixture containing, on the one hand, a compound of nitrogen with oxygen,
termed nitrous anhydride, N_{2}O_{3}, and on the other hand, ammonia,
NH_{3}--that is, a compound of nitrogen with hydrogen. By heating such a
mixture, the oxygen of the nitrous anhydride combines with the hydrogen
of the ammonia, forming water, and gaseous nitrogen is evolved, 2NH_{3}
+ N_{2}O_{3} = 3H_{2}O + N_{4}. Nitrogen is procured by this method in
the following manner:--A solution of caustic potash is saturated with
nitrous anhydride, by which means potassium nitrite is formed. On the
other hand, a solution of hydrochloric acid saturated with ammonia is
prepared; a saline substance called sal-ammoniac, NH_{4}Cl, is thus
formed in the solution. The two solutions thus prepared are mixed
together and heated. Reaction takes place according to the equation
KNO_{2} + NH_{4}Cl = KCl + 2H_{2}O + N_{2}. This reaction proceeds in
virtue of the fact that potassium nitrite and ammonium chloride are
salts which, on interchanging their metals, give potassium chloride
and ammonium nitrite, NH_{4}NO_{2}, which breaks up into water and
nitrogen. This reaction does not take place without the aid of heat,
but it proceeds very easily at a moderate temperature. Of the resultant
substances, the nitrogen only is gaseous. Pure nitrogen may be obtained
by drying the resultant gas and passing it through a solution of
sulphuric acid (to absorb a certain quantity of ammonia which is evolved
in the reaction).[4 bis]
[3] The oxygen compounds of nitrogen (for example, N_{2}O, NO, NO_{2})
are decomposed at a red heat by themselves, and under the action
of red-hot copper, iron, sodium, &c., they give up their oxygen
to the metals, leaving the nitrogen free. According to Meyer and
Langer (1885), nitrous oxide, N_{2}O, decomposes below 900°,
although not completely.
[4] Chlorine and bromine (in excess), as well as bleaching powder
(hypochlorites), take up the hydrogen from ammonia, NH_{3},
leaving nitrogen. Nitrogen is best procured from ammonia by the
action of a solution of sodium hypobromite on solid sal-ammoniac.
[4 bis] Lord Rayleigh in 1894, when determining the weight of a volume
of carefully purified nitrogen by weighing it in one and the same
globe, found that the gas obtained from air, by the action of
incandescent copper (or iron or by removing the oxygen by ferrous
oxide) was always 1/200 heavier than the nitrogen obtained from its
compounds, for instance, from the oxide or suboxide of nitrogen,
decomposed by incandescent pulverulent iron or from the ammonia
salt of nitrous acid. For the nitrogen procured from air, he
obtained, at 0° and 760·4 mm. pressure, a weight = 2·310 grms.,
while for the nitrogen obtained from its compounds, 2·299 grms.
This difference of about 1/200 could not be explained by the
nitrogen not having been well purified, or by inaccuracy of
experiment, and was the means for the remarkable discovery of the
presence of a heavy gas in air, which will be mentioned in Note
16 bis.
Nitrogen is a gaseous substance which does not differ much in physical
properties from air; its density, referred to hydrogen, is approximately
equal to 14--that is, it is slightly lighter than air, its density
referred to air being 0·972; one litre of nitrogen weighs 1·257 gram.
Nitrogen mixed with oxygen, which is slightly heavier than air, forms
air. It is a gas which, like oxygen and hydrogen, is liquefied with
difficulty, and is but little soluble in water and other liquids. Its
absolute boiling point[5] is about -140°; above this temperature it is
not liquefiable by pressure, and at lower temperatures it remains a
gas at a pressure of 50 atmospheres. Liquid nitrogen boils at -193°,
so that it may be employed as a source of great cold. At about -203°,
in vaporising under a decrease of pressure, nitrogen solidifies into
a colourless snow-like mass. Nitrogen does not burn,[5 bis] does not
support combustion, is not absorbed by any of the reagents used in gas
analysis, at least at the ordinary temperature--in a word, it presents
a whole series of negative chemical properties; this is expressed by
saying that this element has no energy for combination. Although it
is capable of forming compounds both with oxygen and hydrogen as well
as with carbon, yet these compounds are only formed under particular
circumstances, to which we will directly turn our attention. At a red
heat nitrogen combines with boron, titanium, and silicon, barium,
magnesium, &c., forming very stable nitrogenous compounds,[6] whose
properties are entirely different from those of nitrogen with hydrogen,
oxygen and carbon. However, the combination of nitrogen with carbon,
although it does not take place directly between the elements at a
red heat, yet proceeds with comparative ease by heating a mixture of
charcoal with an alkaline carbonate, especially potassium carbonate or
barium carbonate, to redness, carbo-nitrides or cyanides of the metals
being formed; for instance, K_{2}CO_{3} + 4C + N_{2} = 2KCN + 3CO.[7]
[5] See Chapter II. Note 29.
[5 bis] See Note 11 bis.
[6] The combination of boron with nitrogen is accompanied by the
evolution of sufficient heat to raise the mass to redness;
titanium combines so easily with nitrogen that it is difficult
to obtain it free from that element; magnesium easily absorbs
nitrogen at a red heat. It is a remarkable and instructive fact
that these compounds of nitrogen are very stable and non-volatile.
Carbon (C = 12) with nitrogen gives cyanogen, C_{2}N_{2}, which
is gaseous and very unstable, and whose molecule is not large,
whilst boron (B = 11) forms a nitrogenous compound which is solid,
non-volatile, and very stable. Its composition, BN, is similar
to that of cyanogen, but its molecular weight, B_{n}N_{n}, is
probably greater. Its composition, like that of N_{2}Mg_{3},
NNa_{3}, N_{2}Hg_{3} and of many of the metallic nitrides,
corresponds to ammonia with the substitution of all its hydrogen
by a metal. In my opinion, a detailed study of the transformations
of the nitrides now known, should lead to the discovery of many
facts in the history of nitrogen.
[7] This reaction, so far as is known, does not proceed beyond a
certain limit, probably because cyanogen, CN, itself breaks up
into carbon and nitrogen.
Nitrogen is found with oxygen in the air, but they do not readily
combine. Cavendish, however, in the last century, showed that _nitrogen
combines with oxygen under the influence of a series of electric
sparks_. Electric sparks in passing through a moist[8] mixture
of nitrogen and oxygen cause these elements to combine, forming
reddish-brown fumes of oxides of nitrogen,[9] which form nitric
acid,[10] NHO_{3}. The presence of the latter is easily recognised,
not only from its reddening litmus paper, but also from its acting as
a powerful oxidiser even of mercury. Conditions similar to these occur
in nature, during a thunderstorm or in other electrical discharges
which take place in the atmosphere; whence it may be taken for granted
that air and rain-water always contain traces of nitric and nitrous
acids.[11] Besides which Crookes (1892) showed that under certain
circumstances and when electricity of high potential[11 bis] passes
through the air, the combination of nitrogen with oxygen is accompanied
by the formation of a true flame. This was also observed previously
(1880) during the passage of electrical discharges through the air.
[8] Frémy and Becquerel took dry air, and observed the formation of
brown vapours of oxides of nitrogen on the passage of sparks.
[9] If a mixture of one volume of nitrogen and fourteen volumes of
hydrogen be burnt, then water and a considerable quantity of
nitric acid are formed. It may be partly due to this that a
certain quantity of nitric acid is produced in the slow oxidation
of nitrogenous substances in an excess of air. This is especially
facilitated by the presence of an alkali with which the nitric
acid formed can combine. If a galvanic current be passed through
water containing the nitrogen and oxygen of the air in solution,
then the hydrogen and oxygen set free combine with the nitrogen,
forming ammonia and nitric acid.
When copper is oxidised at the expense of the air at the ordinary
temperature in the presence of ammonia, oxygen is absorbed, not
only for combination with the copper, but also for the formation
of nitric acid.
The combination of nitrogen with oxygen, even, for example, by
the action of electric sparks, is not accompanied by an explosion
or rapid combination, as in the action of a spark on a mixture
of oxygen and hydrogen. This is explained by the fact that heat
is not evolved in the combination of nitrogen with oxygen, but
is absorbed--an expenditure of energy is required, there is no
evolution of energy. In fact, there will not be the transmission
of heat from particle to particle which occurs in the explosion
of detonating gas. Each spark will aid the formation of a certain
quantity of the compound of oxygen and nitrogen, but will not
excite the same in the neighbouring particles. In other words, the
combination of hydrogen with oxygen is an exothermal reaction, and
the combination of nitrogen with oxygen an endothermal reaction.
A condition particularly favourable for the oxidation of nitrogen
is the explosion of detonating gas and air if the former be
_in excess_. If a mixture of two volumes of detonating gas and
one volume of air be exploded, then one-tenth of the air is
converted into nitric acid, and consequently after the explosion
has taken place there remain only nine-tenths of the volume of
air originally taken. If a large proportion of air be taken--for
instance, four volumes of air to two volumes of detonating
gas--then the temperature of the explosion is lowered, the
volume of air taken remains unchanged, and no nitric acid is
formed. This gives a rule to be observed in making use of the
eudiometer--namely that to weaken the force of the explosion
not less than an equal volume of air should be added to the
explosive mixture. On the other hand a large excess must not be
taken as no explosion would then ensue (_see_ Chapter III. Note
34). Probably in the future means will be found for obtaining
compounds of nitrogen on a large industrial scale by the aid of
electric discharges, and by making use of the inexhaustible mass
of nitrogen in the atmosphere.
[10] In reality nitric oxide, NO, is first formed, but with oxygen and
water it gives (brown fumes) nitrous anhydride, which, as we
shall afterwards learn, in the presence of water and oxygen gives
nitric acid.
[11] The nitric acid contained in the soil, river water (Chapter I.,
Note 2), wells, &c., proceeds (like carbonic anhydride) from the
oxidation of organic compounds which have fallen into water,
soil, &c.
[11 bis] Crookes employed a current of 15 ampères and 65 volts, and
passed it through an induction coil with 330 vibrations per
second, and obtained a flame between the poles placed at a
distance of 46 mm. which after the appearance of the arc and flame
could be increased to 200 mm. A platinum wire fused in the flame.
Further observations showed that under the influence of electrical
discharges,[12] silent as well as with sparks, nitrogen is able to enter
into many reactions with hydrogen and with many hydrocarbons; although
these reactions cannot be effected by exposure to a red heat. Thus,
for instance, a series of electric sparks passed through a mixture of
nitrogen and hydrogen causes them to combine and _form ammonia_[13] or
nitrogen hydride, NH_{3}, composed of one volume of nitrogen and three
volumes of hydrogen. This combination is limited to the formation of
6 per cent. of ammonia, because ammonia is decomposed, although not
entirely (94/100) by electric sparks. This signifies that under the
influence of an electrical discharge the reaction NH_{3} = N + 3H is
reversible, consequently it is a dissociation, and in it a state of
equilibrium is arrived at. The equilibrium may be destroyed by the
addition of gaseous hydrochloric acid, HCl, because with ammonia it
forms a solid saline compound, sal-ammoniac, NH_{4}Cl, which (being
formed from a gaseous mixture of 3H, N, and HCl) fixes the ammonia.
The remaining mass of nitrogen and hydrogen, under the action of the
sparks, again forms ammonia, and in this manner _solid sal-ammoniac is
obtained to the end by the action of a series of electric sparks on a
mixture of gaseous_ N, H_{3}, _and_ HCl.[14] Berthelot (1876) showed
that under the action of a silent discharge many non-nitrogenous organic
substances (benzene, C_{6}H_{6}, cellulose in the form of paper, resin,
glucose, C_{6}H_{10}O_{5}, and others) absorb nitrogen and form complex
nitrogenous compounds, which are capable, like albuminous substances, of
evolving their nitrogen as ammonia when heated with alkalis.[15]
[12] This property of nitrogen, which under normal conditions is
inactive, leads to the idea that under the influence of an
electric discharge gaseous nitrogen changes in its properties;
if not permanently like oxygen (electrolysed oxygen or ozone
does not react on nitrogen, according to Berthelot), it may be
temporarily at the moment of the action of the discharge, just
as some substances under the action of heat are permanently
affected (that is, when once changed remain so--for instance,
white phosphorus passes into red, &c.), whilst others are only
temporarily altered (the dissociation of S_{6} into S_{2} or
of sal-ammoniac into ammonia and hydrochloric acid). Such a
proposition is favoured by the fact that nitrogen gives two kinds
of spectra, with which we shall afterwards become acquainted. It
may be that the molecules N_{2} then give less complex molecules,
N containing one atom, or form a complex molecule N_{3}, like
oxygen in passing into ozone. Probably under a silent discharge
the molecules of oxygen, O_{2}, are partly decomposed and the
individual atoms O combine with O_{2}, forming ozone, O_{3}.
[13] This reaction, discovered by Chabrié and investigated by Thénard,
was only rightly understood when Deville applied the principles
of dissociation to it.
[14] The action of nitrogen on acetylene (Berthelot) resembles this
reaction. A mixture of these gases under the influence of a
silent discharge gives hydrocyanic acid, C_{2}H_{2} + N_{2} =
2CNH. This reaction cannot proceed beyond a certain limit because
it is reversible.
[15] Berthelot successfully employed electricity of even feeble
potential in these experiments, which fact led him to think that
in nature, where the action of electricity takes place very
frequently, a part of the complex nitrogenous substances may
proceed from the gaseous nitrogen of the air by this method.
As the nitrogenous substances of organisms play a very important
part in them (organic life cannot exist without them), and as
the nitrogenous substances introduced into the soil are capable
of invigorating its crops (of course in the presence of the
other nourishing principles required by plants), the question
of the means of converting the atmospheric nitrogen into the
nitrogenous compounds of the soil, or into _assimilable nitrogen_
capable of being absorbed by plants and of forming complex
(albuminous) substances in them, is one of great theoretical and
practical interest. The artificial (technical) conversion of the
atmospheric nitrogen into nitrogenous compounds, notwithstanding
repeated attempts, cannot yet be considered as fulfilled in a
practical remunerative manner although its possibility is already
evident. Electricity will probably aid in solving this very
important practical problem. When the theoretical side of the
question is further advanced, then without doubt an advantageous
means will be found for the manufacture of nitrogenous substances
from the nitrogen of the air; and this is needed, before all,
for the agriculturist, to whom nitrogenous fertilisers form an
expensive item, and are more important than all other manures.
One thousand tons of farmyard manure do not generally contain
more than four tons of nitrogen in the form of complex
nitrogenous substances, and this amount of nitrogen is contained
in twenty tons of ammonium sulphate, therefore the effect of
a mass of farmyard manure in respect to the introduction of
nitrogen may be produced by small quantities of artificial
nitrogenous fertilisers (_see_ Note 15 bis).
By such indirect methods does the gaseous nitrogen of the atmosphere
yield its primary compounds, in which form it enters into plants, and
is elaborated in them into complex albuminous substances.[15 bis] But,
starting from a given compound of nitrogen with hydrogen or oxygen,
we may, without the aid of organisms, obtain, as will afterwards be
partially indicated, most diverse and complex nitrogenous substances,
which cannot by any means be formed directly from gaseous nitrogen. In
this we see an example not only of the difference between an element in
the free state and an intrinsic element, but also of those circuitous
or _indirect methods_ by which substances are formed in nature. The
discovery, prognostication, and, in general, the study of such indirect
methods of the preparation and formation of substances forms one of the
existing problems of chemistry. From the fact that A does not act at all
on B, it must not be concluded that a compound AB is not to be formed.
The substances A and B contain atoms which occur in AB, but their state
or the nature of their motion may not be at all that which is required
for the formation of AB, and in this substance the chemical state of
the elements may be as different as the state of the atoms of oxygen in
ozone and in water. Thus free nitrogen is inactive; but in its compounds
it very easily enters into changes and is distinguished by great
activity. An acquaintance with the compounds of nitrogen confirms this.
But, before entering on this subject, let us consider air as a mass
containing free nitrogen.
[15 bis] Although the numerous, and as far as possible accurate and
varied researches made in the physiology of plants have proved
that the higher forms of plants are not capable of directly
absorbing the nitrogen of the atmosphere and converting it into
complex albuminous substances, still it has been long and
repeatedly observed that the amount of nitrogenous substances in
the soil is increased by the cultivation of plants of the bean
(leguminous) family such as pea, acacia, &c. A closer study of
these plants has shown that this is connected with the formation
of peculiar nodular swellings in their roots caused by the growth
of peculiar micro-organisms (bacteria) which cohabit the soil with
the roots, and are capable of absorbing nitrogen from the air,
_i.e._ of converting it into assimilated nitrogen. This branch of
plant physiology, which forms another proof of the important part
played by micro-organisms in nature, cannot be discussed in this
work, but it should be mentioned, since it is of great theoretical
and practical interest, and, moreover, phenomena of this kind,
which have recently been discovered, promise to explain, to some
extent at least, certain of the complex problems concerning the
development of life on the earth.
Judging from what has been already stated, it will be evident that
_atmospheric air_[16] contains a mixture of several gases and vapours.
Some of them are met with in it in nearly constant proportions, whilst
others, on the contrary, are very variable in their amount. The chief
component parts of air, placed in the order of their relative amounts,
are the following: nitrogen,[16 bis] oxygen, aqueous vapour, carbonic
anhydride, nitric acid, salts of ammonia, oxides of nitrogen, and also
ozone, hydrogen peroxide, and complex organic nitrogenous substances.
Besides these, air generally contains water, as spray, drops, and snow,
and particles of solids, perhaps of cosmic origin in certain instances,
but in the majority of cases proceeding from the mechanical translation
of solid particles from one locality to another by the wind. These
small solid and liquid particles (having a large surface in proportion
to their weight) are suspended in air as solid matter is suspended in
turbid water; they often settle on the surface of the earth, but the
air is never entirely free from them because they are never in a state
of complete rest. Then, air not unfrequently contains incidental traces
of various substances as everyone knows by experience. These incidental
substances sometimes belong to the order of those which act injuriously,
the germs of lower organisms--for instance of moulds--and the class of
carriers of infectious diseases.
[16] Under the name of atmospheric air the chemist and physicist
understand ordinary air containing nitrogen and oxygen only,
notwithstanding that the other component parts of air have a very
important influence on the living matter of the earth's surface.
That air is so represented in science is based on the fact that
only the two components above-named are met with in air in a
constant quantity, whilst the others are variable. The solid
impurities may be separated from air required for chemical or
physical research by simple filtration through a long layer of
cotton-wool placed in a tube. Organic impurities are removed by
passing the air through a solution of potassium permanganate. The
carbonic anhydride contained in air is absorbed by alkalis--best
of all, soda-lime, which in a dry state in porous lumps absorbs
it with exceeding rapidity and completeness. Aqueous vapour
is removed by passing the air over calcium chloride, strong
sulphuric acid, or phosphoric anhydride. Air thus purified is
accepted as containing only nitrogen and oxygen, although in
reality it still contains a certain quantity of hydrogen and
hydrocarbons, from which it may be purified by passing over
copper oxide heated to redness. The copper oxide then oxidises
the hydrogen and hydrocarbons--it burns them, forming water and
carbonic anhydride, which may be removed as above described. When
it is said that in the determination of the density of gases the
weight of air is taken as unity, it is understood to be such air,
containing only nitrogen and oxygen.
[16 bis] Thanks to the remarkable discovery made in the summer of 1894
by Lord Rayleigh and Prof. Ramsay, the well-known component
elements of air must now he supplemented by 1 p.c. (by volume) of
a heavy gas (density about 19, H = 1), inactive like nitrogen,
which was discovered in the researches made by Lord Rayleigh on
the density of nitrogen as mentioned in note 4 bis. Up to the
present time this gas has been always determined together with
nitrogen, because it combines with neither the hydrogen in the
eudiometer nor with the copper in the gravimetric method of
determining the composition of air, and therefore has always
remained with the nitrogen. It has been possible to separate it
from nitrogen since magnesium absorbs nitrogen at a red heat,
while this gas remains unabsorbed, and was found to have a density
nearly one and a half time greater than that of nitrogen (is it
not a polymer of nitrogen, N_{3}?). It is now known also that this
gas gives a luminous spectrum, which contains the bright blue line
observed in the spectrum of nitrogen. Owing to the fact that it is
an exceedingly inert substance, even more so than nitrogen, it has
been termed Argon. Further reference will be made to it in the
Appendix.
In the air of the various countries of the earth, at different
longitudes and at different altitudes above its surface, on the ocean
or on the dry land--in a word, in the air of most diverse localities
of the earth--the oxygen and nitrogen are found everywhere to be in a
constant ratio. This is, moreover, self-evident from the fact that the
air constantly diffuses (intermixes by virtue of the internal motion of
the gaseous particles) and is also put into motion and intermixed by
the wind, by which processes it is equalised in its composition over
the entire surface of the earth. In those localities where the air is
subject to change, and is in a more or less enclosed space, or, at any
rate, in an unventilated space, it may alter very considerably in its
composition. For this reason the air in dwellings, cellars, and wells,
in which there are substances absorbing oxygen, contains less of this
gas, whilst the air on the surface of standing water, which abounds in
the lower orders of plant life evolving oxygen, contains an excess of
this gas.[17] The constant composition of air over the whole surface of
the earth has been proved by a number of most careful researches.[18]
[17] As a further proof of the fact that certain circumstances may
change the composition of air, it will be enough to point out
that the air contained in the cavities of glaciers contains only
up to 10 p.c. of oxygen. This depends on the fact that at low
temperatures oxygen is much more soluble in snow-water and snow
than nitrogen. When shaken up with water the composition of air
should change, because the water dissolves unequal quantities of
oxygen and nitrogen. We have already seen (Chapter I.) that the
air boiled off from water saturated at about 0° contains about
thirty-five volumes of oxygen and sixty-five volumes of nitrogen,
and we have considered the reason of this.
[18] The analysis of air by weight conducted by Dumas and Boussingault
in Paris, which they repeated many times between April 27 and
September 22, 1841, under various conditions of weather, showed
that the amount by weight of oxygen only varies between 22·89
p.c. and 23·08 p.c., the average amount being 23·07 p.c. Brunner,
at Bern in Switzerland, and Bravais, at Faulhorn in the Bernese
Alps, at a height of two kilometres above the level of the sea,
Marignac at Geneva, Lewy at Copenhagen, and Stas at Brussels,
have analysed the air by the same methods, and found that its
composition does not exceed the limits determined for Paris.
The most recent determinations (with an accuracy of ±0·05 p.c.)
confirm the conclusion that the composition of the atmosphere is
constant.
As there are some grounds (which will be mentioned shortly) for
considering that the composition of the air at great altitudes
is slightly different from that at attainable heights--namely,
that it is richer in the lighter nitrogen--several fragmentary
observations made at Munich (Jolly, 1880) gave reason for
thinking that in the upward currents (that is in the region of
minimum barometric pressure or at the centres of meteorological
cyclones) the air is richer in oxygen than in the descending
currents of air (in the regions of anticyclones or of barometric
maxima); but more carefully conducted observations showed
this supposition to be incorrect. Improved methods for the
analysis of air have shown that certain slight variations in
its composition do actually occur, but in the first place they
depend on incidental local influences (on the passage of the air
over mountains and large surfaces of water, regions of forest
and herbage, and the like), and in the second place are limited
to quantities which are scarcely distinguishable from possible
errors in the analyses. The researches made by Kreisler in
Germany (1885) are particularly convincing.
The considerations which lead to the supposition that the
atmosphere at great altitudes contains less oxygen than at the
surface of the earth are based on the law of partial pressures
(Chapter I.) According to this law, the equilibrium of the
oxygen in the strata of the atmosphere is not dependent on
the equilibrium of the nitrogen, and the variation in the
densities of both gases with the height is determined by the
pressure of each gas separately. Details of the calculations
and considerations here involved are contained in my work _On
Barometric Levellings_, 1876, p. 48.
On the basis of the law of partial pressure and of hypsometrical
formulæ, expressing the laws of the variation of pressures at
different altitudes, the conclusion may be deduced that at the
upper strata of the atmosphere the proportion of the nitrogen
with respect to the oxygen increases, but the increase will not
exceed a fraction per cent., even at altitudes of four and a half
to six miles, the greatest height within the reach of men either
by climbing mountains or by means of balloons. This conclusion is
confirmed by the analyses of air collected by Welch in England
during his aëronautic ascents.
_The analysis of air_ is effected by converting the oxygen into a
non-gaseous compound, so as to separate it from the air. The original
volume of the air is first measured, and then the volume of the
remaining nitrogen. The quantity of oxygen is calculated either from
the difference between these volumes or by the weight of the oxygen
compound formed. All the volumetric measurements have to be corrected
for pressure, temperature, and moisture (Chapters I. and II.) The medium
employed for converting the oxygen into a non-gaseous substance should
enable it to be taken up from the nitrogen to the very end without
evolving any gaseous substance. So, for instance,[19] a mixture of
pyrogallol, C_{6}H_{6}O_{3}, with a solution of a caustic alkali absorbs
oxygen with great ease at the ordinary temperature (the solution turns
black), but it is unsuited for accurate analysis because it requires
an aqueous solution of an alkali, and it alters the composition of
the air by acting on it as a solvent.[20] However, for approximate
determinations this simple method gives results which are entirely
satisfactory.
[19] The complete absorption of the oxygen may be attained by
introducing moist phosphorus into a definite volume of air; the
occurrence of this is recognised by the fact of the phosphorus
becoming non-luminous in the dark. The amount of oxygen may be
determined by measuring the volume of nitrogen remaining. This
method however cannot give accurate results, owing to a portion
of the air being dissolved in the water, to the combination
of some of the nitrogen with oxygen and to the necessity of
introducing and withdrawing the phosphorus, which cannot be
accomplished without introducing bubbles of air.
[20] For rapid and approximate analyses (technical and hygienic), such
a mixture is very suitable for determining the amount of oxygen
in mixtures of gases from which the substances absorbed by
alkalis have first been removed. According to certain observers,
this mixture evolves a certain (small) quantity of carbonic oxide
after absorbing oxygen.
The determinations in a eudiometer (Chapter III.) give more exact
results, if all the necessary corrections for changes of pressure,
temperature, and moisture be taken into account. This determination
is carried out essentially as follows:--A certain amount of air is
introduced into the eudiometer, and its volume is determined. About
an equal volume of dry hydrogen is then passed into the eudiometer,
and the volume again determined. The mixture is then exploded, in the
way described for the determination of the composition of water. The
remaining volume of the gaseous mixture is again measured; it will be
less than the second of the previously measured volumes. Out of three
volumes which have disappeared, one belonged to the oxygen and two to
the hydrogen, consequently one-third of the loss of volume indicates the
amount of oxygen contained in the air.[21]
[21] Details of eudiometrical analysis must, as was pointed out in
Chap. III., Note 32, be looked for in works on analytical
chemistry. The same remark applies to the other analytical
methods mentioned in this work. They are only described for the
purpose of showing the diversity of the methods of chemical
research.
The most complete method for the analysis of air, and one which is
accompanied by the least amount of error, consists in the direct
weighing, as far as is possible, of the oxygen, nitrogen, water,
and carbonic anhydride contained in it. For this purpose the air is
first passed through an apparatus for retaining the moisture and
carbonic anhydride (which will be considered presently), and is then
led through a tube which contains shavings of metallic copper and has
been previously weighed. A long layer of such copper heated to redness
absorbs all the oxygen from the air, and leaves pure nitrogen, whose
weight must be determined. This is done by collecting it in a weighed
and exhausted globe, while the amount by weight of oxygen is shown by
the increase in weight of the tube with the copper after the experiment.
[Illustration: FIG. 38.--Dumas and Boussingault's apparatus for the
analysis of air by weight. The globe B contains 10-15 litres. The air is
first pumped out of it, and it is weighed empty. The tube T connected
with it is filled with copper, and is weighed empty of air. It is heated
in a charcoal furnace. When the copper has become red-hot, the stopcock
_r_ (near R) is slightly opened, and the air passes through the vessels
L, containing a solution of potash, _f_, containing solutions and pieces
of caustic potash, which remove the carbonic anhydride from the air,
and then through _o_ and _t_, containing sulphuric acid (which has
been previously boiled to expel dissolved air) and pumice-stone, which
removes the moisture from the air. The pure air then gives up its oxygen
to the copper in T. When the air passes into T the stopcock R of the
globe B is opened, and it becomes filled with nitrogen. When the air
ceases to flow in, the stopcocks are closed, and the globe B and tube T
weighed. The nitrogen is then pumped out of the tube and it is weighed
again. The increase in weight of the tube shows the amount of oxygen,
and the difference of the second and third weighings of the tube, with
the increase in weight of the globe, gives the weight of the nitrogen.]
Air free from moisture and carbonic anhydride[22] contains 20·95 to
20·88[23] parts by volume of oxygen; the mean amount of oxygen will
therefore be 20·92 ± 0·05 per cent. Taking the density of air = 1 and of
oxygen = 1·105 and nitrogen 0·972 the composition of air by weight will
be 23·12 per cent. of oxygen and 76·88 per cent. of nitrogen.[24]
[22] Air free from carbonic anhydride indicates after explosion the
presence of a small quantity of carbonic anhydride, as De
Saussure remarked, and air free from moisture, after being
passed over red-hot copper oxide, appears invariably to contain
a small quantity of water, as Boussingault has observed. These
observations lead to the assumption that air always contains a
certain quantity of gaseous hydrocarbons, like marsh gas, which,
as we shall afterwards learn, is evolved from the earth, marshes,
&c. Its amount, however, does not exceed a few hundredths per
cent.
[23] The analyses of air are accompanied by errors, and there are
variations of composition attaining hundredths per cent.; the
average normal composition of air is therefore only correct to
the first decimal place.
[24] These figures express the mean composition of air from an average
of the most accurate determinations; they are accurate within
±0·05 p.c.
[Illustration: FIG. 39.--Apparatus for the absorption and washing
of gases, known as Liebig's bulbs. The gas enters _m_, presses on
the absorptive liquid, and passes from m into _b_, _c_, _d_, and _e_
consecutively, and escapes through _f_.]
[Illustration: FIG. 40.--Geisler's potash bulbs. The gas enters at
_a_, and passes through a solution of potash in the lower bulbs, where
the carbonic anhydride is absorbed, and the gas escapes from _b_. The
lower bulbs are arranged in a triangle, so that the apparatus can stand
without support.]
The possibility of the composition of air being altered by the mere
action of a solvent very clearly shows that the component parts of air
are in a state of mixture, in which any gases may occur; they do not
in this case form a definite compound, although the composition of the
atmosphere does appear constant under ordinary conditions. The fact that
its composition varies under different conditions confirms the truth
of this conclusion, and therefore the constancy of the composition of
air must not be considered as in any way dependent on the nature of the
gases entering into its composition, but only as proceeding from cosmic
phenomena co-operating towards this constancy. It must be admitted,
therefore, that the processes evolving oxygen, and chiefly the processes
of the respiration of plants, are of equal force with those processes
which absorb oxygen over the entire surface of the earth.[25]
[25] In Chapter III., Note 4, an approximate calculation is made for
the determination of the balance of oxygen in the entire
atmosphere; it may therefore he supposed that the composition of
air will vary from time to time, the relation between vegetation
and the oxygen absorbing processes changes; but as the atmosphere
of the earth can hardly have a definite limit and we have
already seen (Chapter IV., Note 33) that there are observations
confirming this, it follows that our atmosphere should vary in
its component parts with the entire heavenly space, and therefore
it must he supposed that any variation in the composition by
weight of the air can only take place exceedingly slowly, and in
a manner imperceptible by experiment.
[Illustration: FIG. 41.--Tube for the absorption of carbonic acid. A
plug of cotton wool is placed in the bulb to prevent the powder of
soda-lime being carried off by the gas. The tube contains soda-lime and
chloride of calcium.]
Air always contains more or less moisture[26] and _carbonic anhydride_
produced by the respiration of animals and the combustion of carbon and
carboniferous compounds. The latter shows the properties of an acid
anhydride. In order to determine the amount of carbonic anhydride in
air, substances are employed which absorb it--namely, alkalis either in
solution or solid. A solution of caustic potash, KHO, is poured into
light glass vessels, through which the air is passed, and the amount
of carbonic anhydride is determined by the increase in weight of the
vessel. But it is best to take a solid porous alkaline mass such as
soda-lime.[27] With a slow current of air a layer of soda-lime 20 cm.
in length is sufficient to completely deprive 1 cubic metre of air of
the carbonic anhydride it contains. A series of tubes containing calcium
chloride for absorbing the moisture[28] is placed before the apparatus
for the absorption of the carbonic anhydride, and a measured mass of
air is passed through the whole apparatus by means of an aspirator. In
this manner the determination of the moisture is combined with the
absorption of the carbonic anhydride. The arrangement shown in fig. 38
is such a combination.
[26] The amount of moisture contained in the air is considered in
greater detail in the study of physics and meteorology and the
subject has been mentioned above, in Chapter I., Note 1, where
the methods of absorbing moisture from gases were pointed out.
[27] Soda-lime is prepared in the following manner:--Unslaked lime
is finely powdered and mixed with a slightly warmed and very
strong solution of caustic soda. The mixing should be done in
an iron dish, and the materials should be well stirred together
until the lime begins to slake. When the mass becomes hot, it
boils, swells up, and solidifies, forming a porous mass very rich
in alkali and capable of rapidly absorbing carbonic anhydride. A
lump of caustic soda or potash presents a much smaller surface
for absorption and therefore acts much less rapidly. It is
necessary to place an apparatus for absorbing water after the
apparatus for absorbing the carbonic anhydride, because the
alkali in absorbing the latter gives off water.
[28] It is evident that the calcium chloride employed for absorbing the
water should be free from lime or other alkalis in order that it
may not retain carbonic anhydride. Such calcium chloride may be
prepared in the following manner: A perfectly neutral solution of
calcium chloride is prepared from lime and hydrochloric acid; it
is then carefully evaporated first on a water-bath and then on a
sand-bath. When the solution attains a certain strength a scum is
formed, which solidifies at the surface. This scum is collected,
and will be found to be free from caustic alkalis. It is
necessary in any case to test it before use, as otherwise a large
error may be introduced into the results, owing to the presence
of free alkali (lime). It is best to pass carbonic anhydride
through the tube containing the calcium chloride for some time
before the experiment, in order to saturate any free alkali that
may remain from the decomposition of a portion of the calcium
chloride by water, CaCl_{2} + 2H_{2}O = CaOH_{2}O + 2HCl.
The amount of carbonic anhydride[29] in free air is incomparably more
constant than the amount of moisture. The average amount in 100 volumes
of dry air is approximately 0·03 volume--that is, 10,000 volumes of air
contain about three volumes of carbonic anhydride, most frequently about
2·95 volumes. As the specific gravity of carbonic anhydride referred to
air = 1·52, it follows that 100 parts by weight of air contain 0·045
part by weight of carbonic anhydride. This quantity varies according
to the time of year (more in winter), the altitude above the level of
the sea (less at high altitudes), the proximity to forests and fields
(less) or cities (greater), &c. But the variation is small and rarely
exceeds the limits of 2-1/2 to 4 ten-thousandths by volume.[30] As there
are many natural local influences which either increase the amount of
carbonic anhydride in the air (respiration, combustion, decomposition,
volcanic eruptions, &c.), or diminish it (absorption by plants and
water), the reason of the great constancy in the amount of this gas in
the air must be looked for, in the first place, in the fact that the
wind mixes the air of various localities together, and, in the second
place, in the fact that the waters of the ocean, holding carbonic acid
in solution,[31] form an immense reservoir for regulating the amount of
this gas in the atmosphere. Immediately the partial pressure of the
carbonic anhydride in the air decreases, the water evolves it, and when
the partial pressure increases, it absorbs it, and thus nature supplies
the conditions for a natural state of moving equilibrium in this as in
so many other instances.[32]
[29] Recourse is had to special methods when the determination only
takes note of the carbonic anhydride of the air. For instance,
it is absorbed by an alkali which does not contain carbonates
(by a solution of baryta or caustic soda mixed with baryta),
and then the carbonic anhydride is expelled by an excess of an
acid, and its amount determined by the volume given off. A rapid
method of determining CO_{2} (for hygienic purposes) is given by
the fall of tension produced by the introduction of an alkali
(the air having been either brought to dryness or saturated
with moisture). Dr. Schidloffsky's apparatus is based upon this
principle. The question as to the amount of carbonic anhydride
present in the air has been submitted to many voluminous and
exact researches, especially those of Reiset, Schloesing, Müntz,
and Aubin, who showed that the amount is not subject to such
variations as at first announced on the basis of incomplete and
insufficiently accurate determinations.
[30] It is a different case in enclosed spaces, in dwellings, cellars,
wells, caves, and mines, where the renewal of air is impeded.
Under these circumstances large quantities of carbonic anhydride
may accumulate. In cities, where there are many conditions for
the evolution of carbonic anhydride (respiration, decomposition,
combustion), its amount is greater than in free air, yet even
in still weather the difference does not often exceed one
ten-thousandth (that is, rarely attains 4 instead of 2·9 vols. in
10000 vols. of air).
[31] In the sea as well as in fresh water, carbonic acid occurs in
two forms, directly dissolved in the water, and combined with
lime as calcium bicarbonate (hard waters sometimes contain very
much carbonic acid in this form). The tension of the carbonic
anhydride in the first form varies with the temperature, and
its amount with the partial pressure, and that in the form of
acid salts is under the same conditions, for direct experiments
have shown a similar dependence in this case, although the
quantitative relations are different in the two cases.
[32] In studying the phenomena of nature the conclusion is arrived at
that the universally reigning state of mobile equilibrium forms
the chief reason for that harmonious order which impresses all
observers. It not unfrequently happens that we do not see the
causes regulating the order and harmony; in the particular
instance of carbonic anhydride, it is a striking circumstance
that in the first instance a search was made for an harmonious
and strict uniformity, and in incidental (insufficiently
accurate and fragmentary) observations conditions were even
found for concluding it to be absent. When, later, the rule of
this uniformity was confirmed, then the causes regulating such
order were also discovered. The researches of Schloesing were of
this character. Deville's idea of the dissociation of the acid
carbonates of sea-water is suggested in them. In many other cases
also, a correct interpretation can only follow from a detailed
investigation.
Besides nitrogen, oxygen, moisture, and carbonic acid, all the other
substances occurring in air are found in infinitesimally small
quantities by weight, and therefore the _weight of a cubic measure of
air_ depends, to a sensible degree, on the above-named components alone.
We have already mentioned that at 0° and 760 mm. pressure the weight
of a cubic litre of air is 1·293 gram. This weight varies with the
acceleration of gravity, _g_, so that if _g_ be expressed in metres the
weight of a litre of air, _e_ = _g_ × 0·131844 gram. For St. Petersburg
_g_ is about 9·8188, and therefore _e_ is about 1·2946,[33] the air
being understood to be dry and free from carbonic anhydride. Taking
the amount of the latter as 0·03 per 100 volumes, we obtain a greater
weight; for example, for St. Petersburg _e_ = 1·2948 instead of 1·2946
gram. The weight of one litre of moist air in which the tension[34] of
the aqueous vapour (partial pressure) = _f_ mm., at a pressure (total)
of air of H millimetres, at a temperature _t_, will be (_i.e._, if
at 0° and 760 mm. the weight of dry air = _e_) equal to _e_/(1 +
0·00367_t_) × (H - 0·38_f_)/760. For instance, if H = 730 mm., _t_ = 20°,
and _f_ = 10 mm. (the moisture is then slightly below 60 p.c.), the
weight of a litre of air at St. Petersburg = 1·1527 gram.[35]
[33] The difference of the weight of a litre of dry air (free from
carbonic anhydride) at 0° and 760 mm., at different longitudes
and altitudes, depends on the fact that the force of gravity
varies under these conditions, and with it the pressure of the
barometrical column also varies. This is treated in detail
in my works _On the Elasticity of Gases_ and _On Barometric
Levellings_, and 'The Publications of the Weights and Measures
Department' (_Journal of the Russian Physico-Chemical Society_,
1894).
In reality the weight is not measured in absolute units of weight
(in pressure--refer to works on mechanics and physics), but in
relative units (grams, scale weights) whose mass is invariable,
and therefore the variation of the weight of the weights itself
with the change of gravity must not be here taken into account,
for we are here dealing with weights proportional to masses,
since with a change of locality the weight of the weights varies
as the weight of a given volume of air does. In other words: the
mass of a substance always remains constant, but the pressure
produced by it varies with the acceleration of gravity: the gram,
pound, and other units of weight are really units of mass.
[34] The tension of the aqueous vapour in the air is determined by
hygrometers and other similar methods. It may also be determined
by analysis (_see_ Chapter I., Note 1).
[35] For rapid calculation the weight of a litre of air (in a room) in
St. Petersburg, may under these conditions (H, _t_, and _f_)
be obtained by the formula _e_ = 1·20671 + 0·0016 [H_{1}-755 +
2·6(18°-_t_°)] where H_{1} = H-0·38_f_. In determining the weight
of small and heavy objects (crucibles, &c. in analysis, and in
determining the specific gravities of liquids, &c.) _a correction
may be introduced for the loss of weight_ in the air of the
room, by taking the weight of a litre of air displaced as 1·2
gram, and consequently 0·0012 gram for every cubic centimetre.
But if gases or, in general, large vessels are weighed, and the
weighings require to be accurate, it is necessary to take into
account all the data for the determination of the density of the
air (_t_, H, and _f_), because sensitive balances can determine
the possible variations of the weight of air, as in the case of a
litre the weight of air varies in centigrams, even at a constant
temperature, with variations of H and _f_. Some time ago (1859) I
proposed the following method and applied it for this purpose. A
large light and closed vessel is taken, and its volume and weight
in a vacuum are accurately determined, and verified from time
to time. On weighing it we obtain the weight in air of a given
density, and by subtracting this weight from its absolute weight
and dividing by its volume we obtain the density of the air.
The presence of ammonia, a compound of nitrogen and hydrogen, in the
air, is indicated by the fact that all acids exposed to the air absorb
ammonia from it after a time. De Saussure observed that aluminium
sulphate is converted by air into a double sulphate of ammonium and
aluminium, or the so-called ammonia alum. Quantitative determinations
have shown that the amount of ammonia[36] contained in air varies at
different periods. However, it may be accepted that 100 cubic metres of
air do not contain less than 1 or more than 5 milligrams of ammonia.
It is remarkable that mountain air contains more ammonia than the air
of valleys. The air in those places where animal substances undergoing
change are accumulated, and especially that of stables, generally
contains a much greater quantity of this gas. This is the reason of the
peculiar pungent smell noticed in such places. Moreover ammonia, as we
shall learn in the following chapter, combines with acids, and should
therefore be found in air in the form of such combinations, since air
contains carbonic and nitric acids.
[36] Schloesing studied the equilibrium of the ammonia of the
atmosphere and of the rivers, seas, &c., and showed that the
amount of the gas is interchangeable between them. The ratio
between the amount of ammonia in a cubic metre of air and a litre
of water at 0° = 0·004, at 10° = 0·010, at 25° = 0·040 to 1, and
therefore in nature there is a state of equilibrium in the amount
of ammonia in the atmosphere and waters.
The presence of nitric acid in air is proved without doubt by the fact
that rain-water contains an appreciable amount of it.
Further (as already mentioned in Chapter IV.), air contains ozone
and hydrogen peroxide and nitrous acid (and its ammonia salt), _i.e._
substances having a direct oxidising action (for instance, upon iodized
starch-paper), but they are present in very small quantities.[37]
[37] Whilst formed in the air these oxidising substances (N_{2}O_{3},
ozone and hydrogen peroxide) at the same time rapidly disappear
from it by oxidising those substances which are capable of
being oxidised. Owing to this instability their amounts vary
considerably, and, as would be expected, they are met with to an
appreciable amount in pure air, whilst their amount decreases
to zero in the air of cities, and especially in dwellings where
there is a maximum of substances capable of oxidisation and a
minimum of conditions for the formation of such bodies. There
is a causal connection between the amount of these substances
present in the air and its purity--that is, the amount of foreign
residues of organic origin liable to oxidation present in the
air. Where there is much of such residues their amount must be
small. When they are present the amount of organic substances
must be small, as otherwise they would be destroyed. For this
reason efforts have been made to apply ozone for purifying the
air by evolving it by artificial means in the atmosphere; for
instance, by passing a series of electrical sparks through
the ventilating pipes conveying air into a building. Air thus
ozonised destroys by oxidation--that is, brings about the
combustion of--the organic residues present in the air, and
thus will serve for purifying it. For these reasons the air of
cities contains less ozone and such like oxidising agents than
country air. This forms the distinguishing feature of country
air. However, animal life cannot exist in air containing a
comparatively large amount of ozone.
Besides substances in a gaseous or vaporous state,[38] there is always
found a more or less considerable quantity of substances which are
not known in a state of vapour. These substances are present in the
air as _dust_. If a linen surface, moistened with an acid, be placed
in perfectly pure air, then the washings are found to contain sodium,
calcium, iron, and potassium.[39] Linen moistened with an alkali absorbs
carbonic, sulphuric, phosphoric, and hydrochloric acids. Further, the
presence of organic substances in air has been proved by a similar
experiment. If a glass globe be filled with ice and placed in a room
where are a number of people, then the presence of organic substances,
like albuminous substances, may be proved in the water which condenses
on the surface of the globe. It may be that the miasmas causing
infection in marshy localities, hospitals, and in certain epidemic
illnesses proceed from the presence of such substances in the air (and
especially in water, which contains many micro-organisms), as well as
from the presence of germs of lower organisms in the air as a minute
dust. Pasteur proved the existence of such germs in the air by the
following experiment:--He placed gun-cotton (pyroxylin), which has the
appearance of ordinary cotton, in a glass tube. Gun-cotton is soluble
in a mixture of ether and alcohol, forming the so-called collodion.
A current of air was passed through the tube for a long period of
time, and the gun-cotton was then dissolved in a mixture of ether and
alcohol. An insoluble residue was thus obtained which actually contained
the germs of organisms, as was shown by microscopical observations,
and by their capacity to develop into organisms (mould, &c.) under
favourable conditions. The presence of these germs determines the
property of air of bringing about the processes of putrefaction and
fermentation--that is the fundamental alteration of organic substances,
which is accompanied by an entire change in their properties. The
appearance of lower organisms, both vegetable and animal, is always to
be remarked in these processes. Thus, for instance, in the process of
fermentation, when, for example, wine is procured from the sweet juice
of grapes, a sediment separates out which is known under the name of
lees, and contains peculiar yeast organisms. Germs are required before
these organisms can appear.[40] They are floating in the air, and fall
into the sweet fermentable liquid from it. Finding themselves under
favourable conditions, the germs develop into organisms; they are
nourished at the expense of the organic substance, and during growth
change and destroy it, and bring about fermentation and putrefaction.
This is why, for instance, the juice of the grape when contained in the
skin of the fruit, which allows access of the air but is impenetrable to
the germs, does not ferment, does not alter so long as the skin remains
intact. This is also the reason why animal substances when kept from the
access of air may be preserved for a great length of time. Preserved
foods for long sea voyages are kept in this way.[41] Hence it is
evident that however infinitesimal the quantity of germs carried in the
atmosphere may be, still they have an immense significance in nature.[42]
[38] Amongst them we may mention iodine and alcohol, C_{2}H_{6}O, which
Müntz found to be always present in air, the soil, and water,
although in minute traces only.
[39] A portion of the atmospheric dust is of cosmic origin; this is
undoubtedly proved by the fact of its containing metallic iron as
do meteorites. Nordenskiöld found iron in the dust covering snow,
and Tissandier in every kind of air, although naturally in very
small quantities.
[40] The idea of the spontaneous growth of organisms in a suitable
medium, although still upheld by many, has since the work of
Pasteur and his followers (and to a certain extent of his
predecessors) been discarded, because it has been proved how,
when, and whence (from the air, water, &c.) the germs appear;
that fermentation as well as infectious diseases cannot take
place without them; and chiefly because it has been shown that
any change accompanied by the development of the organisms
introduced may be brought about at will by the introduction of
the germs into a suitable medium.
[41] In further confirmation of the fact that putrefaction and
fermentation depend on germs carried in the air, we may cite the
circumstance that poisonous substances destroying the life of
organisms stop or hinder the appearance of the above processes.
Air which has been heated to redness or passed through sulphuric
acid no longer contains the germs of organisms, and loses the
faculty of producing fermentation and putrefaction.
[42] Their presence in the air is naturally due to the diffusion of
germs into the atmosphere, and owing to their microscopical
dimensions, they, as it were, hang in the air in virtue of their
large surfaces compared to their weight. In Paris the amount of
dust suspended in the air equals from 6 (after rain) to 23 grams
per 1,000 c.m. of air.
Thus we see that air contains a great variety of substances. The
nitrogen, which is found in it in the largest quantity, has the least
influence on those processes which are accomplished by the action
of air. The oxygen, which is met with in a lesser quantity than the
nitrogen, on the contrary takes a very important part in a number of
reactions; it supports combustion and respiration, it brings about
decomposition and every process of slow oxidation. The part played by
the moisture of air is well known. The carbonic anhydride, which is met
with in still smaller quantities, has an immense significance in nature,
inasmuch as it serves for the nourishment of plants. The importance of
the ammonia and nitric acid is very great, because they are the sources
of the nitrogenous substances comprising an indispensable element in all
living organisms. And, lastly, the infinitesimal quantity of germs also
have a great significance in a number of processes. Thus it is not the
quantitative but the qualitative relations of the component parts of the
atmosphere which determine its importance in nature.[43]
[43] We see similar cases everywhere. For example, the predominating
mass of sand and clay in the soil takes hardly any chemical part
in the economy of the soil in respect to the nourishment of
plants. The plants by their roots search for substances which
are diffused in comparatively small quantities in the soil. If a
large quantity of these nourishing substances are removed, then
the plants will not develop in the soil, just as animals die in
oxygen.
Air, being a mixture of various substances, may suffer considerable
_changes_ in consequence of incidental circumstances. It is particularly
necessary to remark those changes in the composition of air which take
place in dwellings and in various localities where human beings have
to remain during a lengthy period of time. The respiration of human
beings and animals alters the air.[44] A similar deterioration of air
is produced by the influence of decomposing organic substances, and
especially of substances burning in it.[45] Hence it is necessary
to have regard to the purification of the air of dwellings. The
renewal of air, the replacing of respired by fresh air, is termed
'ventilation,'[46] and the removal of foreign and injurious admixtures
from the air is called 'disinfection.'[47] The accumulation of all kinds
of impurities in the air of dwellings and cities is the reason why the
air of mountains, forests, seas, and non-marshy localities, covered with
vegetation or snow, is distinguished for its freshness, and, in all
respects, beneficial action.
[44] A man in breathing burns about 10 grams of carbon per hour--that
is, he produces about 880 grams, or (as 1 cub.m. of carbonic
anhydride weighs about 2,000 grams) about 5/12 c.m. of carbonic
anhydride. The air coming from the lungs contains 4 p.c. of
carbonic anhydride by volume. The exhaled air acts as a direct
poison, owing to this gas and to other impurities.
[45] For this reason candles, lamps, and gas change the composition
of air almost in the same way as respiration. In the burning of 1
kilogram of stearin candles, 50 cubic metres of air are changed
as by respiration--that is, 4 p.c. of carbonic anhydride will
be formed in this volume of air. The respiration of animals and
exhalations from their skins, and especially from the intestines
and the excrements and the transformations taking place in them,
contaminate the air to a still greater extent, because they
introduce other volatile substances besides carbonic anhydride
into the air. At the same time that carbonic anhydride is formed
the amount of oxygen in the air decreases, and there is noticed
the appearance of miasmata which occur in but small quantity,
but which are noticeable in passing from fresh air into a
confined space full of such adulterated air. The researches of
Schmidt and Leblanc and others show that even with 20·6 p.c. of
oxygen (instead of 20·9 p.c.), when the diminution is due to
respiration, air becomes noticeably less fit for respiration,
and that the heavy feeling experienced in such air increases
with a lesser percentage of oxygen. It is difficult to remain
for a few minutes in air containing 17·2 p.c. of oxygen. These
observations were chiefly obtained by observations on the air of
different mines, at different depths below the surface. The air
of theatres and buildings full of people also proves to contain
less oxygen; it was found on one occasion that at the end of a
theatrical representation the air in the stalls contained 20·75
p.c. of oxygen, whilst the air at the upper part of the theatre
contained only 20·36 p.c. The amount of carbonic anhydride in
the air may be taken as a measure of its purity (Pettenkofer).
When it reaches 1 p.c. it is very difficult for human beings to
remain long in such air, and it is necessary to set up a vigorous
ventilation for the removal of the adulterated air. In order
to keep the air in dwellings in a uniformly good state, it is
necessary to introduce at least 10 cubic metres of fresh air per
hour per person. We saw that a man exhales about five-twelfths
of a cubic metre of carbonic anhydride per day. Accurate
observations have shown that air containing one-tenth p.c. of
exhaled carbonic anhydride (and consequently also a corresponding
amount of the other substances evolved together with it) is not
felt to be oppressive; and therefore the five-twelfth cubic
metres of carbonic anhydride should be diluted with 420 cubic
metres of fresh air if it be desired to keep not more than
one-tenth p.c. (by volume) of carbonic anhydride in the air.
Hence a man requires 420 cubic metres of air per day, or 18 cubic
metres per hour. With the introduction of only 10 cubic metres of
fresh air per person, the amount of carbonic anhydride may reach
one-fifth p.c., and the air will not then be of the requisite
freshness.
[46] The _ventilation_ of inhabited buildings is most necessary, and
is even indispensable in hospitals, schools, and similar
buildings. In winter it is carried on by the so-called
calorifiers or stoves heating the air before it enters. The best
kind of calorifiers in this respect are those in which the fresh
cold air is led through a series of pipes heated by the hot gases
coming from a stove. In ventilation, particularly during winter,
care is taken that the incoming air shall be moist, because
in winter the amount of moisture in the air is very small.
Ventilation, besides introducing fresh air into a dwelling-place,
must also withdraw the air already spoilt by respiration and
other causes--that is, it is necessary to construct channels for
the escape of the bad air, besides those for the introduction of
fresh air. In ordinary dwelling-places, where not many people
are congregated, the ventilation is conducted by natural means,
in the heating by fires, through crevices, windows, and various
orifices in walls, doors, and windows. In mines, factories, and
workrooms ventilation is of the greatest importance.
Animal vitality may still continue for a period of several
minutes in air containing up to 30 p.c. of carbonic anhydride, if
the remaining 70 p.c. consist of ordinary air; but respiration
ceases after a certain time, and death may even ensue. The
flame of a candle is very easily extinguished in an atmosphere
containing from 5 to 6 p.c. of carbonic anhydride, but animal
vitality can be sustained in it for a somewhat long time,
although the effect of such air is exceedingly painful even to
the lower animals. There are mines in which a lighted candle
easily goes out from the excess of carbonic anhydride, but in
which the miners have to remain for a long time. The presence
of 1 p.c. of carbonic oxide is deadly even to cold-blooded
animals. The air in the galleries of a mine where blasting
has taken place, is known to produce a state of insensibility
resembling that produced by charcoal fumes. Deep wells and
vaults not unfrequently contain similar substances, and their
atmosphere often causes suffocation. The atmospheres of such
places cannot be tested by lowering a lighted candle into it, as
these poisonous gases would not extinguish the flame. This method
only suffices to indicate the amount of carbonic anhydride. If a
candle keeps alight, it signifies that there is less than 6 p.c.
of this gas. In doubtful cases it is best to lower a dog or other
animal into the air to be tested. If CO_{2} be very carefully
added to air, the flame of a candle is not extinguished (although
it becomes very much smaller) even when the gas amounts to 12
p.c. of air. Researches made by F. Clowes (1894) show that the
flames (in every case 3/4 in. long) of different combustible
substances are extinguished by the gradual addition of different
percentages of nitrogen and carbonic acid to the air; the
percentage sufficient to extinguish the flame being as follows
(the percentage of oxygen is given in parenthesis):
p.c. CO_{2} p.c. N.
Absolute alcohol 14 (18·1) 21 (16·6)
Candle 14 (18·1) 22 (16·4)
Hydrogen 58 ( 8·8) 70 ( 6·3)
Coal gas 33 (14·1) 46 (11·3)
Carbonic oxide 24 (16·0) 28 (15·1)
Methane 10 (18·9) 17 (17·4)
The flames of all solid and liquid substances is extinguished by
almost the same percentage of CO_{2} or N_{2}, but the flames of
different gases vary in this respect, and hydrogen continues to
burn in mixtures which are far poorer in oxygen than those in
which the flames of other combustible gases are extinguished;
the flame of methane CH_{4} is the most easily extinguished. The
percentage of nitrogen may be greater than that of CO_{2}. This,
together with the fact that, under the above circumstances, the
flame of a gas before going out becomes fainter and increases in
size, seems to indicate that the chief reason for the extinction
of the flame is the fall in its temperature.
[47] Different so-called disinfectants purify the air, and prevent the
injurious action of certain of its components by changing or
destroying them. Disinfection is especially necessary in those
places where a considerable amount of volatile substances
are evolved into the air, and where organic substances are
decomposed; for instance, in hospitals, closets, &c. The numerous
disinfectants are of the most varied nature. They may be divided
into oxidising, antiseptic, and absorbent substances. To the
oxidising substances used for disinfection belong chlorine, and
various substances evolving it, because chlorine in the presence
of water oxidises the majority of organic substances, and this
is why chlorine is used as a disinfectant for Siberian plagues.
Further, to this class belong the permanganates of the alkalis
and peroxide of hydrogen, as substances easily oxidising matters
dissolved in water; these salts are not volatile like chlorine,
and therefore act much more slowly, and in a much more limited
sphere. Antiseptic substances are those which convert organic
substances into such as are little prone to change, and prevent
putrefaction and fermentation. They most probably kill the
germs of organisms occurring in miasmata. The most important of
these substances are creosote and phenol (carbolic acid), which
occur in tar, and act in preserving smoked meat. Phenol is a
substance little soluble in water, volatile, oily, and having the
characteristic smell of smoked objects. Its action on animals in
considerable quantities is injurious, but in small quantities,
used in the form of a weak solution, it prevents the change of
animal matter. The smell of privies, which depends on the change
of excremental matter, may be easily removed by means of chlorine
or phenol. Salicylic acid, thymol, common tar, and especially
its solution in alkalis as proposed by Nensky, &c., are also
substances having the same property. Absorbent substances are
of no less importance, especially as preventatives, than the
preceding two classes of disinfectants, inasmuch as they are
innocuous. They are those substances which absorb the odoriferous
gases and vapours emitted during putrefaction, which are chiefly
ammonia, sulphuretted hydrogen, and other volatile compounds. To
this class belong charcoal, certain salts of iron, gypsum, salts
of magnesia, and similar substances, as well as peat, mould, and
clay. Questions of disinfection and ventilation appertain to the
most serious problems of common life and hygiene. These questions
are so vast that we are here able only to give a short outline of
their nature.