and that by any process we reduce it to a combination of oxygen and
ozone occupying ninety-five cubic inches. Now, if the mercury absorbed
the ozone, and we found, say, that there only remained eighty-five
cubic inches of oxygen, we could reason in this way:—Ten cubic inches
were occupied by the ozone before the mercury absorbed it; but these
correspond to fifteen cubic inches of oxygen; hence, ozone must be
denser than oxygen in the proportion of fifteen to ten, or three to
two. And whatever result might have followed, a real absorption of the
ozone by the mercury would have satisfactorily solved the problem.
But the result actually obtained did not admit of interpretation in
this way. The apparent absorption of the ozone by the mercury, that
is, the disappearance of the ozone from the mixture, was accompanied
by _no diminution of volume at all_. In other words, returning to
our illustrative case, after the absorption of the ozone from the
ninety-five cubic inches occupied by the mixture, there still remained
ninety-five cubic inches of oxygen; so that it seemed as though an
evanescent volume of ozone corresponded in weight to five cubic inches
of oxygen. This solution, of course, could not be admitted, since it
made the density of ozone _infinite_.
The explanation of this perplexing experiment is full of interest and
instruction. The following is the account given by Mr. C. W. Heaton
(Professor of Chemistry at Charing Cross Hospital), slightly modified,
however, so that it may be more readily understood.
Modern chemists adopt, as a convenient mode of representing the
phenomena which gases exhibit, the theory that every gas, whether
elementary or compound, consists of minute molecules. They suppose
that these molecules are of equal size, and are separated by equal
intervals so long as the gas remains unchanged in heat and density.
This view serves to account for the features of resemblance presented
by all gases. The features in which gases vary are accounted for by the
theory that the molecules are differently constituted. The molecules
are supposed to be clusters of atoms, and the qualities of a gas are
assumed to depend on the nature and arrangement of these ultimate
atoms. The molecules of some elements consist but of a single atom;
the molecules of others are formed by pairs of atoms; those of others
by triplets; and so on. Again, the molecules of compound gases are
supposed to consist of combinations of different _kinds_ of atoms.
Now, Dr. Odling, to whom we owe the solution of the perplexing problem
described above, thus interpreted the observed phenomena. A molecule
of oxygen contains two atoms, one of ozone contains three, _and the
oxidizing power of ozone depends on the ease with which it parts with
its third atom of oxygen_. Thus, in the experiment which perplexed
Messrs. Andrews and Tait, the mercury only _seemed_ to absorb the
ozone; in reality it converted the ozone into oxygen by removing
its third atom. And now we see how to interpret such a result as we
considered in our illustrative case. Five cubic inches of oxygen
gave up their atoms, each atom combining with one of the remaining
oxygen doublets, so as to form a set of ozone triplets. Clearly, then,
fifteen cubic inches of oxygen were transformed into ozone. They now
occupied but ten cubic inches; so that the mixture, or ozonized oxygen,
contained eighty-five cubic inches of oxygen and ten of ozone. When the
mercury was introduced, it simply transformed all the ozone triplets
into oxygen doublets, by taking away the odd atom from each. It thus
left ten cubic inches of oxygen, which, with the remaining eighty-five,
constituted the ninety-five cubic inches observed to remain after the
supposed absorption of the ozone.
It follows, of course, that ozone is half as heavy again as oxygen.
But, as Mr. Heaton remarked, “this beautiful hypothesis, although
accounting perfectly for all known facts, was yet but a probability.
One link was lacking in the chain of evidence, and that link M. Soret
has supplied by a happily devised experiment.” Although mercury and
most substances are only capable of converting ozone into oxygen, oil
of turpentine has the power of absorbing ozone in its entirety. Thus,
when the experiment was repeated, with oil of turpentine in place of
the mercury, the ozone was absorbed, and the remaining oxygen, instead
of occupying ninety-five inches, occupied but eighty-five. After
this, no doubt could remain that Dr. Odling’s ingeniously conceived
hypothesis was the correct explanation of Messrs. Andrews and Tait’s
experiment.
We recognize, then, in ozone a sort of concentrated oxygen, with this
peculiar property, that it possesses an extraordinary readiness to
part with its characteristic third atom, and so disappear _as ozone_,
two-thirds of its weight remaining as oxygen.
It is to this peculiarity that ozone owes the properties which render
it so important to our welfare. We are indeed, as yet, in no position
to theorize respecting this element, our knowledge of its very
existence being so recent, and our information respecting its presence
in our atmosphere being of still more recent acquisition.
Indeed, it is well remarked by Mr. Heaton, that we had, until quite
lately, no reason for confidently adopting Schönbein’s view that ozone
exists in our atmosphere. The test-papers which Schönbein made use of
turned blue under the influence of ozone, it is true, but they were
similarly influenced by other elements which are known to exist in our
atmosphere, and even the sun’s rays turned them blue. However, Dr.
Andrews has shown how the character of the air producing the change
can be further tested, so as to render it certain that ozone only has
been at work. If air which colours the test-papers be found to lose
the property after being heated, the change can only be due to ozone,
because nitrous and nitric acids (which have the power of colouring the
test-papers) would not be removed by the heat, whereas ozone is changed
by heat into oxygen.
Once we are certain that ozone exists in the air, we must recognize
the fact that its presence cannot fail to have an important bearing on
our health and comfort; for ozone is an exceedingly active agent, and
cannot exist anywhere without setting busily to its own proper work.
What that work is, and whether it is beneficial or deleterious to
ourselves, remains to be considered.
In the first place, ozone has immense power as a disinfectant.
It decomposes the products emanating from putrefying matter more
effectually than any other known element. Perhaps the most striking
proof ever given of its qualities in this respect is that afforded by
an experiment conducted by Dr. Richardson a few years ago.
He placed a pint of blood taken from an ox in a large wide-mouthed
bottle. The blood had then coagulated, and it was left exposed to
the air until it had become entirely redissolved by the effects of
decomposition. At the end of a year the blood was put into a stoppered
bottle, and set aside for seven years. “The bottle was then taken from
its hiding-place,” says Dr. Richardson, “and an ounce of the blood was
withdrawn. The fluid was so offensive as to produce nausea when the
gases evolved from it were inhaled. It was subjected by Dr. Wood and
myself to a current of ozone. For a few minutes the odour of ozone
was destroyed by the odour of the gases from the blood; gradually the
offensive smell passed away; then the fluid mass became quite sweet,
and at last a faint odour of ozone was detected, whereupon the current
was stopped. The blood was thus entirely deodorized; but another and
most singular phenomenon was observed. The dead blood coagulated as the
products of decomposition were removed, and this so perfectly, that
from the new clot that was formed serum exuded. Before the experiment
commenced, I had predicted on theoretical grounds that secondary
coagulation would follow on purification; and this experiment, as well
as several others afterwards performed, verified the truth of the
prediction.”
It will of course be understood that ozone, in thus acting as a
disinfectant, is transformed into oxygen. It parts with its third atom
as in the mercury experiment, and so loses its distinctive peculiarity.
Thus we might be led to anticipate the results which come next to be
considered.
Ozone has certain work to do, and in doing that work is transmuted into
oxygen. It follows, then, that where there has been much work for ozone
to do, there we shall find little ozone left in the air. Hence, in
open spaces where there is little decomposing matter, we should expect
to find more ozone than in towns or cities. This accords with what is
actually observed. And not only is it found that country air contains
more ozone than town air, but it is found that air which has come from
the sea has more ozone than even the country air, while air in the
crowded parts of large cities has no ozone at all, nor has the air of
inhabited rooms.
So far as we have gone, we might be disposed to speak unhesitatingly in
favour of the effects produced by ozone. We see it purifying the air
which would otherwise be loaded by the products of decomposing matter,
we find it present in the sea air and the country air which we know
to be so bracing and health-restoring after a long residence in town,
and we find it absent just in those places which we look upon as most
unhealthy.
Again, we find further evidence of the good effects of ozone in
the fact that cholera and other epidemics never make their dreaded
appearance in the land when the air is well supplied with ozone—or
in what the meteorologists call “the ozone-periods.” And though we
cannot yet explain the circumstance quite satisfactorily, we yet seem
justified in ascribing to the purifying and disinfecting qualities of
ozone our freedom at those times from epidemics to which cleanliness
and good sanitary regulations are notedly inimical.
But there is a reverse side to the picture. And as we described an
experiment illustrating the disinfecting qualities of ozone before
describing the good effects of the element, we shall describe an
experiment illustrating certain less pleasing qualities of ozone,
before discussing the deleterious influences which it seems capable of
exerting.
Dr. Richardson found that when the air of a room was so loaded with
ozone as to be only respirable with difficulty, animals placed in the
room were affected in a very singular manner. “In the first place,”
he says, “all the symptoms of nasal catarrh and of irritation of the
mucous membranes of the nose, the mouth, and the throat were rapidly
induced. Then followed free secretion of saliva and profuse action of
the skin—perspiration. The breathing was greatly quickened, and the
action of the heart increased in proportion.” When the animals were
suffered to remain yet longer within the room, congestion of the lungs
followed, and the disease called by physicians “congestive bronchitis”
was set up.
A very singular circumstance was noticed also as to the effects of
ozone on the different orders of animals. The above-mentioned effects,
and others which accompanied them, the description of which would be
out of place in these pages, were developed more freely in carnivorous
than in herbivorous animals. Rats, for example, were much more easily
influenced by ozone than rabbits were.
The results of Dr. Richardson’s experiments prepare us to hear that
ozone-periods, though characterized by the absence of certain diseases,
bring with them their own forms of disease. Apoplexy, epilepsy,
and other similar diseases seem peculiarly associated with the
ozone-periods, insomuch that eighty per cent. of the deaths occurring
from them take place on days when ozone is present in the air in larger
quantities than usual. Catarrh, influenza, and affections of the
bronchial tubes, also affect the ozone-periods.
We see, then, that we have much yet to learn respecting ozone before
we can pronounce definitively whether it is more to be welcomed or
dreaded. We must wait until the researches which are in progress have
been carried out to their conclusion, and perhaps even then further
modes of inquiry will have to be pursued before we can form a definite
opinion.
_DEW._
There are few phenomena of common occurrence which have proved more
perplexing to philosophers than those which attend the deposition of
dew. Every one is familiar with these phenomena, and in very early
times observant men had noticed them; yet it is but quite recently
that the true theory of dew has been put forward and established.
This theory affords a striking evidence of the value of careful and
systematic observation applied even to the simplest phenomena of nature.
It was observed, in very early times, that dew is only formed on
clear nights, when, therefore, the stars are shining. It was natural,
perhaps, though hardly philosophical, to conclude that dew is directly
shed down upon the earth from the stars; accordingly, we find the
reference of dew to stellar influences among the earliest theories
propounded in explanation of the phenomenon.
A theory somewhat less fanciful, but still depending on supposed
stellar influences, was shortly put forward. It was observed that
dew is only formed when the atmosphere is at a low temperature; or,
more correctly, when the air is at a much lower temperature than has
prevailed during the daytime. Combining this peculiarity with the
former ancient philosophers reasoned in the following manner: Cold
generates dew, and dew appears only when the skies are clear—that is,
when the stars are shining; hence it follows that the stars generate
cold, and thus lead indirectly to the formation of dew. Hence arose
the singular theory, that as the sun pours down heat upon the earth, so
the stars (and also the moon and planets) pour down cold.
Nothing is more common—we may note in passing—than this method of
philosophizing, especially in all that concerns weather-changes; and
perhaps it would be impossible to find a more signal instance of the
mistakes into which men are likely to fall when they adopt this false
method of reasoning; for, so far is it from being true that the stars
shed cold upon the earth, that the exact reverse is the case. It has
been established by astronomers and physicists that an important
portion of the earth’s heat-supply is derived from the stars.
Following on these fanciful speculations came Aristotle’s theory
of dew—celebrated as one of the most remarkable instances of the
approximation which may sometimes be made to the truth by clever
reasoning on insufficient observations. For we must not fall into
the mistake of supposing, as many have done, that Aristotle framed
hypotheses without making observations; indeed, there has seldom lived
a philosopher who has made more observations than he did. His mistake
was that he extended his observations too widely, not making enough on
each subject. He imagined that, by a string of syllogisms, he could
make a few supply the place of many observations.
Aristotle added two important facts to our knowledge respecting
dew—namely, first, that dew is only formed in serene weather; and
secondly, that it is not formed on the summits of mountains. Modern
observations show the more correct statement of the case to be that dew
is _seldom_ formed either in windy weather or on the tops of mountains.
Now, Aristotle reasoned in a subtle and able manner on these two
observations. He saw that dew must be the result of processes which
are interfered with when the air is agitated, and which do not extend
high above the earth’s surface; he conjectured, therefore, that dew is
simply caused by the discharge of vapour from the air. “Vapour is a
mixture,” he said, “of water and heat, and as long as water can get a
supply of heat, vapour rises. But vapour cannot rise high, or the heat
would get detached from it; and vapour cannot exist in windy weather,
but becomes dissipated. Hence, in high places, and in windy weather,
dew cannot be formed for want of vapour.” He derided the notion that
the stars and moon cause the precipitation of dew. “On the contrary,
the sun,” he said, “is the cause; since its heat raises the vapour,
from which the dew is formed when that heat is no longer present to
keep up the vapour.”
Amidst much that is false, there is here a good deal that is sound.
The notion that heat is some substance which floats up the vapour,
and may become detached from it in high or windy places, is of course
incorrect. So also is the supposition that the dew is produced by
the _fall_ of condensed vapour as the heat passes away. Nor is it
correct to say that the absence of the sun causes the condensation of
vapour, since, as we shall presently see, the cold which causes the
deposition of dew results from more than the mere absence of the sun.
But, in pointing out that the discharge of vapour from the air, owing
to loss of heat, is the true cause of the deposition of dew, Aristotle
expressed an important truth. It was when he attempted to account for
the discharge that he failed. It will be observed, also, that his
explanation does not account for the observed fact that dew is only
formed in clear weather.
Aristotle’s views did not find acceptance among the Greeks or Romans;
they preferred to look on the moon, stars, and planets as the agents
which cause the deposition of dew. “This notion,” says a modern author,
“was too beautiful for a Greek to give up, and the Romans could not do
better than follow the example of their masters.”
In the middle ages, despite the credit attached to Aristotle’s name,
those who cultivated the physical sciences were unwilling to accept his
views; for the alchemists (who alone may be said to have been students
of nature) founded their hopes of success in the search for the
philosopher’s stone, the _elixir vitæ_, and the other objects of their
pursuit, on occult influences supposed to be exercised by the celestial
bodies. It was unlikely, therefore, that they would willingly reject
the ancient theory which ascribed dew to lunar and stellar radiations.
But at length Baptista Porta adduced evidence which justified him in
denying positively that the moon or stars exercise any influence on the
formation of dew. He discovered that dew is sometimes deposited on the
inside of glass panes; and again, that a bell-glass placed over a plant
in cold weather is more copiously covered with dew within than without;
nay, he observed that even some opaque substances show dew on their
_under_ surface when none appears on the upper. Yet, singularly enough,
Baptista Porta rejected that part of Aristotle’s theory which was alone
correct. He thought his observations justified him in looking on dew
as condensed—not from vapour, as Aristotle thought—but from the air
itself.
But now a new theory of dew began to be supported. We have seen that
not only the believers in stellar influence, but Aristotle also, looked
on dew as falling from above. Porta’s experiments were opposed to this
view. It seemed rather as if dew rose from the earth. Observation also
showed that the amount of dew obtained at different heights from the
ground diminishes with the height. Hence, the new theorists looked upon
dew as an exhalation from the ground and from plants—a fine steam, as
it were, rising upwards, and settling principally on the under surfaces
of objects.
But this view, like the others, was destined to be overthrown.
Muschenbroek, when engaged in a series of observations intended to
establish the new view, made a discovery which has a very important
bearing on the theory of dew: he found that, instead of being deposited
with tolerable uniformity upon different substances,—as falling rain
is, for instance, and as the rising rain imagined by the new theorists
ought to be,—dew forms very much more freely on some substances than
on others.
Here was a difficulty which long perplexed physicists. It appeared that
dew neither fell from the sky nor arose from the earth. The object
itself on which the dew was formed seemed to play an important part in
determining the amount of deposition.
At length it was suggested that Aristotle’s long-neglected explanation
might, with a slight change, account for the observed phenomena. The
formation of dew was now looked upon as a discharge of vapour from the
air, this discharge not taking place necessarily upwards or downwards,
but always from the air next to the object. But it was easy to test
this view. It was understood that the coldness of the object, as
compared with the air, was a necessary element in the phenomenon. It
followed, that if a cold object is suddenly brought into warm air,
there ought to be a deposition of moisture upon the object. This was
found to be the case. Any one can readily repeat the experiment. If a
decanter of ice-cold water is brought into a warm room, in which the
air is not dry—a crowded room, for example—the deposition of moisture
is immediately detected by the clouding of the glass. But there is, in
fact, a much simpler experiment. When we breathe, the moisture in the
breath generally continues in the form of vapour. But if we breathe
upon a window-pane, the vapour is immediately condensed, because the
glass is considerably colder than the exhaled air.
But although this is the correct view, and though physicists had made
a noteworthy advance in getting rid of erroneous notions, yet a theory
of dew still remained to be formed; for it was not yet shown how the
cold, which causes the deposition of dew, is itself occasioned. The
remarkable effects of a clear sky and serene weather in encouraging the
formation of dew, were also still unaccounted for. On the explanation
of these and similar points, the chief interest of the subject depends.
Science owes the elucidation of these difficulties to Dr. Wells, a
London physician, who studied the subject of dew in the commencement
of the present century. His observations were made in a garden three
miles from Blackfriars Bridge.
Wells exposed little bundles of wool, weighing, when dry, ten grains
each, and determined by their increase in weight the amount of moisture
which had been deposited upon them. At first, he confined himself to
comparing the amount of moisture collected on different nights. He
found that although it was an invariable rule that cloudy nights were
unfavourable to the deposition of dew, yet that on some of the very
clearest and most serene nights, less dew was collected than on other
occasions. Hence it became evident that mere clearness was not the only
circumstance which favoured the deposition of dew. In making these
experiments, he was struck by results which appeared to be anomalous.
He soon found that these anomalies were caused by any obstructions
which hid the heavens from his wool-packs: such obstructions hindered
the deposition of dew. He tried a crucial experiment. Having placed
a board on four props, he laid a piece of wool _on_ the board,
and another _under_ it. During a clear night, he found that the
difference in the amount of dew deposited on the two pieces of wool
was remarkable: the upper one gained fourteen grains in weight, the
lower one gained only four grains. He made a little roof over one piece
of wool, with a sheet of pasteboard; and the increase of weight was
reduced to two grains, while a piece of wool outside the roof gained no
less than sixteen grains in weight.
Leaving these singular results unexplained for a while, Dr. Wells
next proceeded to test the temperature near his wool-packs. He found
that where dew is most copiously produced, there the temperature is
lowest. Now, since it is quite clear that the deposition of dew was not
the cause of the increased cold—for the condensation of vapour is a
process _producing heat_—it became quite clear that the formation of
dew is dependent on and proportional to the loss of heat.