Chapter V: Nitrogen and Air (2)

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.