Chapter V: Part 5

27. H. Mache (62) thinks that the ionization observed in the
atmosphere may be wholly accounted for by the radioactive emanation.
If this is true we should have q = [alpha] n^2, where q is the number
of ions of one sign made in 1 cc. of air per second by the emanation,
[alpha] the constant of recombination, and n the number of ions found
simultaneously by, say, Ebert's apparatus. Mache and R. Holfmann, from
observations on the amplitude of saturation currents, deduce q = 4 as
a mean value. Taking for [alpha] Townsend's value 1.2 X 10^(-6), Mache
finds n = 1800. The charge on an ion being 3.4 X 10^(-10) Mache
deduces for the ionic charge, I+ or I-, per cubic metre 1800 X 3.4 X
10^(-10) X 10^6, or 0.6. This is at least of the order observed, which
is all that can be expected from a calculation which assumes I+ and I-
equal. If, however, Mache's views were correct, we should expect a
much closer connexion between I and A than has actually been observed.

28. C.T.R. Wilson (63) seems disposed to regard the action of rainfall
as the most probable source of the negative charge on the earth's
surface. That great separation of positive and negative electricity
sometimes takes place during rainfall is undoubted, and the charge
brought to the ground seems preponderatingly negative. The difficulty
is in accounting for the continuance in extensive fine weather
districts of large positive charges in the atmosphere in face of the
processes of recombination always in progress. Wilson considers that
convection currents in the upper atmosphere would be quite inadequate,
but conduction may, he thinks, be sufficient alone. At barometric
pressures such as exist between 18 and 36 kilometres above the ground
the mobility of the ions varies inversely as the pressure, whilst the
coefficient of recombination [alpha] varies approximately as the
pressure. If the atmosphere at different heights is exposed to
ionizing radiation of uniform intensity the rate of production of ions
per cc., q, will vary as the pressure. In the steady state the number,
n, of ions of either sign per cc. is given by n = [root](q/[alpha]),
and so is independent of the pressure or the height. The conductivity,
which varies as the product of n into the mobility, will thus vary
inversely as the pressure, and so at 36 kilometres will be one hundred
times as large as close to the ground. Dust particles interfere with
conduction near the ground, so the relative conductivity in the upper
layers may be much greater than that calculated. Wilson supposes that
by the fall to the ground of a preponderance of negatively charged
rain the air above the shower has a higher positive potential than
elsewhere at the same level, thus leading to large conduction currents
laterally in the highly conducting upper layers.

29. _Thunder._--Trustworthy frequency statistics for an individual
station are obtainable only from a long series of observations, while
if means are taken from a large area places may be included which
differ largely amongst themselves. There is the further complication
that in some countries thunder seems to be on the increase. In
temperate latitudes, speaking generally, the higher the latitude the
fewer the thunderstorms. For instance, for Edinburgh (64) (1771 to
1900) and London (65) (1763 to 1896) R.C. Mossman found the average
annual number of thunderstorm days to be respectively 6.4 and 10.7;
while at Paris (1873-1893) E. Renou (66) found 27.3 such days. In some
tropical stations, at certain seasons of the year, thunder is almost a
daily occurrence. At Batavia (18) during the epoch 1867-1895, there
were on the average 120 days of thunder in the year.

As an example of a large area throughout which thunder frequency
appears fairly uniform, we may take Hungary (67). According to the
statistics for 1903, based on several hundred stations, the average
number of days of thunder throughout six subdivisions of the country,
some wholly plain, others mainly mountainous, varied only from 21.1 to
26.5, the mean for the whole of Hungary being 23.5. The antithesis of
this exists in the United States of America. According to A.J. Henry
(68) there are three regions of maximum frequency: one in the
south-east, with its centre in Florida, has an average of 45 days of
thunder in the year; a second including the middle Mississippi valley
has an average of 35 days; and a third in the middle Missouri valley
has 30. With the exception of a narrow strip along the Canadian
frontier, thunderstorm frequency is fairly high over the whole of the
United States to the east of the 100th meridian. But to the west of
this, except in the Rocky Mountain region where storms are numerous,
the frequency steadily diminishes, and along the Pacific coast there
are large areas where thunder occurs only once or twice a year.

30. The number of thunderstorm days is probably a less exact measure
of the relative _intensity_ of thunderstorms than statistics as to the
number of persons killed annually by lightning per million of the
population. Table X. gives a number of statistics of this kind. The
letter M stands for "Midland."

TABLE X.--_Deaths by Lightning, per annum, per million Inhabitants._

Hungary 7.7 Upper Missouri and Plains 15
Netherlands 2.8 Rocky Mountains and Plateau 10
England, N. M. 1.8 South Atlantic 8
" E. 1.3 Central Mississippi 7
" S. M. 1.2 Upper " 7
" York and W. M. 1.1 Ohio Valley 7
" N. 1.0 Middle Atlantic 6
Wales 0.9 Gulf States 5
England, S. E. 0.8 New England 4
" N. W. 0.7 Pacific Coast <1*
" S. W. 0.6 North and South Dakota 20
London 0.1 California 0

* Note in case of Pacific coast, Table X., "<1" means "less than 1."

The figure for Hungary is based on the seven years 1897-1903; that for
the Netherlands, from data by A.J. Monne (69) on the nine years
1882-1890. The English data, due to R. Lawson (70), are from
twenty-four years, 1857-1880; those for the United States, due to
Henry (68), are for five years, 1896-1900. In comparing these data
allowance must be made for the fact that danger from lightning is much
greater out of doors than in. Thus in Hungary, in 1902 and 1903, out
of 229 persons killed, at least 171 were killed out of doors. Of the
229 only 67 were women, the only assignable explanation being their
rarer employment in the fields. Thus, _ceteris paribtis_, deaths from
lightning are much more numerous in a country than in an industrial
population. This is well brought out by the low figure for London. It
is also shown conspicuously in figures given by Henry. In New York
State, where the population is largely industrial, the annual deaths
per million are only three, but of the agricultural population eleven.
In states such as Wyoming and the Dakotas the population is largely
rural, and the deaths by lightning rise in consequence. The frequency
and intensity of thunderstorms are unquestionably greater in the Rocky
Mountain than in the New England states, but the difference is not so
great as the statistics at first sight suggest.

TABLE XI.--_Annual Variation of Thunderstorms._

+--------------+------+------+------+------+------+------+------+------+------+------+------+------+
| | Jan. | Feb. | Mar. | Apr. | May. | June | July | Aug. | Sep. | Oct. | Nov. | Dec. |
+--------------+------+------+------+------+------+------+------+------+------+------+------+------+
| Ediburgh | 1.8 | 1.4 | 1.4 | 3.8 | 12.3 | 20.8 | 28.2 | 19.1 | 7.0 | 2.3 | 1.1 | 0.8 |
| London | 0.6 | 0.5 | 1.6 | 6.6 | 12.7 | 18.3 | 25.5 | 19.2 | 9.3 | 3.1 | 1.7 | 0.9 |
| Paris | 0.2 | 0.4 | 2.3 | 7.5 | 14.9 | 21.6 | 22.0 | 17.0 | 9.9 | 3.5 | 0.4 | 0.4 |
| Netherlands | 2.2 | 1.8 | 3.7 | 6.5 | 14.0 | 14.7 | 15.6 | 14.7 | 10.3 | 10.1 | 3.8 | 2.5 |
| France | 2.2 | 2.8 | 4.1 | 8.4 | 13.8 | 18.7 | 14.6 | 13.5 | 10.0 | 6.3 | 3.1 | 2.4 |
| Switzerland | 0.2 | 0.3 | 0.5 | 4.9 | 11.9 | 22.9 | 29.9 | 18.0 | 9.8 | 1.1 | 0.3 | 0.2 |
| Hungary (a) | 0.0 | 0.1 | 1.6 | 5.7 | 20.9 | 25.0 | 23.2 | 15.9 | 5.7 | 1.3 | 0.4 | 0.2 |
| " (b) | 0.0 | 0.0 | 1.0 | 3.2 | 11.8 | 20.6 | 30.7 | 25.3 | 6.9 | 0.5 | 0.0 | 0.0 |
| United States| 0.1 | 0.1 | 1.2 | 4.0 | 14.3 | 25.0 | 27.2 | 20.4 | 5.8 | 1.4 | 0.3 | 0.1 |
| Hong-Kong | 0.0 | 2.1 | 4.3 | 8.5 | 12.8 | 23.4 | 14.9 | 21.3 | 10.6 | 2.1 | 0.0 | 0.0 |
| Trevandrum | 3.2 | 3.8 | 13.1 | 20.9 | 18.6 | 4.9 | 1.2 | 3.5 | 2.5 | 12.9 | 12.0 | 3.3 |
| Batavia | 10.4 | 9.2 | 11.1 | 10.5 | 7.9 | 5.5 | 4.3 | 3.8 | 5.4 | 8.8 | 12.2 | 10.9 |
+--------------+------+------+------+------+------+------+------+------+------+------+------+------+

31. Even at the same place thunderstorms vary greatly in intensity and
duration. Also the times of beginning and ending are difficult to
define exactly, so that several elements of uncertainty exist in data
as to the seasonal or diurnal variation. The monthly data in Table XI.
are percentages of the total for the year. In most cases the figures
are based on the number of days of thunder at a particular station, or
at the average station of a country; but the second set for Hungary
relates to the number of lightning strokes causing fire, and the
figures for the United States relate to deaths by lightning. The data
for Edinburgh, due to R.C. Mossman (64), refer to 130 years, 1771 to
1900. The data for London (1763-1896) are also due to Mossman (65);
for Paris (1873-1893) to Renou (66); for the Netherlands (1882-1900)
to A.J. Monne (69); for France(71) (1886-1899) to Frou and Hann; for
Switzerland to K. Hess (72); for Hungary (67) (1896-1903) to L. von
Szalay and others; for the United States (1890-1900) to A.J. Henry
(68); for Hong-Kong (73) (1894-1903) to W. Doberck. The Trevandrum
(74) data (1853-1864) were due originally to A. Broun; the Batavia
data (1867-1895) are from the Batavia _Observations_, vol. xviii.

Most stations in the northern hemisphere have a conspicuous maximum at
midsummer with little thunder in winter. Trevandrum (8 deg. 31' N.)
and Batavia (6 deg. 11' S.), especially the former, show a double
maximum and minimum.

TABLE XII.--_Diurnal Variation of Thunderstorms._

+--------------------+-----+-----+-----+-----+-----+------+------+------+------+------+------+-------+
| Hour. | 0-2.| 2-4.| 4-6.| 6-8.|8-10.|10-12.|0'-2'.|2'-4'.|4'-6'.|6'-8'.|8'-10'|10'-12'|
+--------------------+-----+-----+-----+-----+-----+------+------+------+------+------+------+-------+
| Finland (76) | 2.3 | 2.0 | 2.2 | 3.0 | 4.6 | 12.1 | 18.9 | 19.2 | 16.1 | 10.1 | 6.1 | 3.4 |
| Edinburgh (64) | 1.7 | 2.0 | 1.4 | 1.7 | 4.7 | 14.2 | 22.4 | 23.7 | 11.9 | 9.2 | 5.1 | 2.0 |
| Belgium (77) | 3.0 | 2.9 | 1.7 | 1.8 | 2.0 | 6.4 | 12.9 | 21.6 | 19.4 | 15.8 | 8.4 | 4.1 |
| Brocken (78) | 1.6 | 2.5 | 1.3 | 1.3 | 4.2 | 3.1 | 12.1 | 28.6 | 22.4 | 10.1 | 7.2 | 5.6 |
| Switzerland (72) | 3.1 | 2.3 | 2.1 | 1.6 | 2.0 | 7.3 | 13.8 | 20.9 | 20.8 | 14.6 | 8.0 | 3.5 |
| Italy (77) | 1.3 | 1.6 | 1.4 | 2.0 | 3.0 | 8.5 | 19.5 | 26.5 | 16.6 | 9.8 | 8.3 | 1.5 |
| Hungary (i.) (67) | 2.1 | 1.9 | 1.9 | 2.1 | 2.9 | 11.5 | 18.1 | 22.0 | 17.9 | 10.7 | 6.2 | 2.8 |
| " (ii.) (67) | 6.9 | 4.2 | 2.3 | 2.0 | 2.0 | 5.0 | 9.9 | 16.9 | 18.2 | 10.7 | 11.7 | 10.0 |
| " (iii.) (75)| 2.3 | 1.9 | 2.0 | 2.4 | 2.7 | 7.9 | 16.1 | 22.1 | 19.1 | 12.7 | 7.6 | 3.2 |
| " (iv.) (75) | 2.6 | 2.2 | 1.9 | 1.9 | 3.6 | 13.3 | 19.9 | 20.7 | 15.2 | 9.2 | 6.2 | 3.3 |
| Trevandrum (74) | 5.6 | 4.9 | 4.3 | 1.3 | 1.4 | 2.0 | 13.3 | 24.5 | 15.9 | 13.3 | 7.6 | 5.9 |
| Agustia (74) | 2.9 | 2.9 | 0.3 | 0.0 | 1.7 | 2.9 | 15.1 | 36.1 | 22.2 | 9.3 | 4.6 | 2.0 |
+--------------------+-----+-----+-----+-----+-----+------+------+------+------+------+------+-------+

32. _Daily Variation._--The figures in Table XII. are again
percentages. They are mostly based on data as to the hour of
commencement of thunderstorms. Data as to the hour when storms are
most severe would throw the maximum later in the day. This is
illustrated by the first two sets of figures for Hungary (67). The
first set relate as usual to the hour of commencement, the second to
the hours of occurrence of lightning causing fires. Of the two other
sets of figures for Hungary (75), (iii.) relates to the central plain,
(iv.) to the mountainous regions to north and south of this. The hour
of maximum is earlier for the mountains, thunder being more frequent
there than in the plains between 8 A.M. and 4 P.M., but less frequent
between 2 and 10 P.M. Trevandrum (8 deg. 31' N., 76 deg. 59' E., 195
ft. above sea-level) and Agustia (8 deg. 37' N., 77 deg. 20' E., 6200
ft. above sea-level) afford a contrast between low ground and high
ground in India. In this instance there seems little difference in the
hour of maximum, the distinguishing feature being the great
concentration of thunderstorm occurrence at Agustia between noon and 6
P.M.

TABLE XIII.

+------+-------------+--------+---------+-------+
| Year.| Netherlands.| France.| Hungary.| U.S.A.|
+------+-------------+--------+---------+-------+
| 1882 | 98 | .. | 141 | .. |
| 1883 | 117 | .. | 195 | .. |
| 1884 | 95 | .. | 229 | .. |
| 1885 | 93 | .. | 192 | .. |
| 1886 | 102 | 251 | 319 | .. |
| 1887 | 78 | 292 | 236 | .. |
| 1888 | 94 | 286 | 232 | .. |
| 1889 | 126 | 294 | 258 | .. |
| 1890 | 93 | 299 | 265 | .. |
| 1891 | 98 | 317 | 302 | 204 |
| 1892 | 86 | 324 | 350 | 251 |
| 1893 | 102 | 288 | 233 | 209 |
| 1894 | 111 | 300 | 333 | 336 |
| 1895 | 119 | 309 | 280 | 426 |
| 1896 | 109 | 266 | 299 | 341 |
| 1897 | 119 | 297 | 350 | 362 |
| 1898 | 95 | 299 | 386 | 367 |
| 1899 | 112 | 299 | 368 | 563 |
| 1900 | 108 | .. | 401 | 713 |
| 1901 | .. | .. | 502 | .. |
| 1902 | .. | .. | 322 | .. |
| 1903 | .. | .. | 256 | .. |
+------+-------------+--------+---------+-------+

33. Table XIII. gives some data as to the variability of thunder from
year to year. The figures for the Netherlands (69) and France (71) are
the number of days when thunder occurred somewhere in the country. Its
larger area and more varied climate give a much larger number of days
of thunder to France. Notwithstanding the proximity of the two
countries, there is not much parallelism between the data. The figures
for Hungary (67) give the number of lightning strokes causing fire;
those for the United States (68) give the number of persons killed by
lightning. The conspicuous maximum in 1901 and great drop in 1902 in
Hungary are also shown by the statistics as to the number of days of
thunder. This number at the average station of the country fell from
38.4 in 1901 to 23.1 in 1902. On the whole, however, the number of
destructive lightning strokes and of days of thunder do not show a
close parallelism.

TABLE XIV.

+----------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
| Decade ending | 1810| 1820| 1830| 1840| 1850| 1860| 1870| 1880| 1890| 1900|
+----------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
| Edinburgh | 4.9| 5.7| 7.7| 6.7| 5.7| 6.5| 5.4| 10.6| 9.4| 9.2|
| London | 9.5| 8.3| 11.5| 11.8| 10.5| 11.9| 9.6| 15.7| 13.0| .. |
| Tilsit | .. | .. | 12.5| 12.1| 16.1| 15.3| 11.9| 17.6| 21.8| .. |
| Germany, South | .. | .. | .. | .. | .. | 49 | 66 | 91 | 143 | 175 |
| " West | .. | .. | .. | .. | .. | 92 | 106 | 187 | 244 | 331 |
| " North | .. | .. | .. | .. | .. | 124 | 135 | 245 | 288 | 352 |
| " East | .. | .. | .. | .. | .. | 102 | 143 | 186 | 210 | 273 |
| " Whole | .. | .. | .. | .. | .. | 90 | 116 | 189 | 254 | 318 |
+----------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+

34. Table XIV. deals with the variation of thunder over longer
periods. The data for Edinburgh (64) and London (65) due to Mossman,
and those for Tilsit, due to C. Kassner (79), represent the average
number of days of thunder per annum. The data for Germany, due to O.
Steffens (80), represent the average number of houses struck by
lightning in a year per million houses; in the first decade only seven
years (1854-1860) are really included. Mossman thinks that the
apparent increase at Edinburgh and London in the later decades is to
some extent at least real. The two sets of figures show some
corroborative features, notably the low frequency from 1860 to 1870.
The figures for Germany--representing four out of six divisions of
that country--are remarkable. In Germany as a whole, out of a million
houses the number struck per annum was three and a half times as great
in the decade 1890 to 1900 as between 1854 and 1860. Von Bezold (81)
in an earlier memoir presented data analogous to Steffens', seemingly
accepting them as representing a true increase in thunderstorm
destructiveness. Doubts have, however, been expressed by others--e.g.
A. Gockel, _Das Gewitter_, p. 106--as to the real significance of the
figures. Changes in the height or construction of buildings, and a
greater readiness to make claims on insurance offices, may be
contributory causes.

35. The fact that a considerable number of people sheltering under
trees are killed by lightning is generally accepted as a convincing
proof of the unwisdom of the proceeding. When there is an option
between a tree and an adjacent house, the latter is doubtless the
safer choice. But when the option is between sheltering under a tree
and remaining in the open it is not so clear. In Hungary (67), during
the three years 1901 to 1903, 15% of the total deaths by lightning
occurred under trees, as against 57% wholly in the open. In the United
States (68) in 1900, only 10% of the deaths where the precise
conditions were ascertained occurred under trees, as against 52% in
the open. If then the risk under trees exceeds that in the open in
Hungary and the United States, at least five or six times as many
people must remain in the open as seek shelter under trees. An
isolated tree occupying an exposed position is, it should be
remembered, much more likely to be struck than the average tree in the
midst of a wood. A good deal also depends on the species of tree. A
good many years' data for Lippe (82) in Germany make the liability to
lightning stroke as follows--the number of each species being supposed
the same:--Oak 57, Fir 39, Pine 5, Beech 1. In Styria, according to K.
Prohaska (83), the species most liable to be struck are oaks, poplars
and pear trees; beech trees again are exceptionally safe. It should,
however, be borne in mind that the apparent differences between
different species may be partly a question of height, exposure or
proximity to water. A good deal may also depend on the soil. According
to Hellmann, as quoted by Henry (82), the liability to lightning
stroke in Germany may be put at chalk 1, clay 7, sand 9, loam 22.

36. Numerous attempts have been made to find periodic variations in
thunderstorm frequency. Among the periods suggested are the 11-year
sun-spot period, or half this (cf. v. Szalay (67)). Ekholm and
Arrhenius (84) claim to have established the existence of a tropical
lunar period, and a 25.929-day period; while P. Polis (85) considers a
synodic lunar period probable. A.B. MacDowall (86) and others have
advanced evidence in favour of the view that thunderstorms are most
frequent near new moon and fewest near full moon. Much more evidence
would be required to produce a general acceptance of any of the above
periods.

37. _St Elmo's Fire._--Luminous discharges from masts, lightning
conductors, and other pointed objects are not very infrequent,
especially during thunderstorms. On the Sonnblick, where the
phenomenon is common, Elster and Geitel (87) have found St Elmo's fire
to answer to a discharge sometimes of positive sometimes of negative
electricity. The colour and appearance differ in the two cases, red
predominating in a positive, blue in a negative discharge. The
differences characteristic of the two forms of discharge are described
and illustrated in Gockel's _Das Gewitter_. Gockel states (l.c. p. 74)
that during snowfall the sign is positive or negative according as the
flakes are large or are small and powdery. The discharge is not
infrequently accompanied by a sizzling sound.

38. Of late years many experiments have been made on the influence of
electric fields or currents on plant growth. S. Lemstrom (88), who was
a pioneer in this department, found an electric field highly
beneficial in some but not in all cases. Attempts have been made to
apply electricity to agriculture on a commercial scale, but the exact
measure of success attained remains somewhat doubtful. Lemstrom
believed atmospheric electricity to play an important part in the
natural growth of vegetation, and he assigned a special role to the
needles of fir and pine trees.

BIBLIOGRAPHY.--The following abbreviations are here used:--M.Z.,
_Meteorologische Zeitschrift_; P.Z., _Physikalische Zeitschrift_; S.,
_Sitzungsberichte k. Akad. Wiss. Wien, Math. Naturw. Klasse_, Theil
ii. 2; P.T., "Philosophical Transactions Royal Society of London";
T.M., _Terrestrial Magnetism_, edited by Dr L.A. Bauer.

Text-books:--(1) G. le Cadet, _Etude du champ electrique de
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Bureau Bulletin_, No. 30, 1901; (69) M.Z., vol. 19, 1902, p. 297; (70)
_Q.J.R. Met. Soc._, vol. 15, 1889, p. 140; (71) M.Z., vol. 20, 1903,
p. 227; (72) M.Z., vol. 20, 1903, p. 522; (73) M.Z., vol. 23, 1906, p.
367; (74) M.Z., vol. 22, 1905, p. 175; (75) J. Hegyfoky, M.Z., vol.
20, 1903, p. 218; (76) M.Z., vol. 22, 1905, p. 575; (77) S. Arrhenius,
M.Z., vol. 5, 1888, p. 348; (78) G. Hellmann, M.Z., vol. 22, 1905, p.
223; (79) M.Z., vol. 11, 1894, p. 239; (80) M.Z., vol. 23, 1906, p.
468; (81) _Berlin Sitz._, 1889, No. 16; (82) A.J. Henry, _U.S. Dept.
of Agriculture Bull._, No. 26, 1899; (83) M.Z., vol. 16, 1899, p. 128;
(84) _K. Sven. Vet. Akad. Hand._, Bd. 19, No. 8, Bd. 20, No. 6, Bd.
31, Nos. 2 and 3; (85) M.Z., vol. 11, 1894, p. 230; (86) _Nature_,
vol. 65, 1902, p. 367; (87) M.Z., vol. 8, 1891, p. 321; (88) _Brit.
Assoc. Report_ for 1898, p. 808, also _Electricity in Agriculture and
Horticulture_ (London, 1904). (C. Ch.)

FOOTNOTE:

[1] see _Authorities_ below.

ATMOSPHERIC RAILWAY. About 1840-1845 great interest was excited by a method of propelling railway trains through the agency of atmospheric pressure. Various inventors worked at the realization of this idea. On the system worked out in England by Jacob Samuda and S. Clegg, a continuous pipe or main was laid between the rails, and in it a partial vacuum was maintained by means of air pumps. A piston fitting closely in it was connected to the leading vehicle of the train by an iron plate which passed through a longitudinal groove or aperture running the whole length of the pipe. This aperture was covered by a valve consisting of a continuous strip of leather, strengthened on each side with iron plates; one edge was fastened, while the other was free to rise, and was closed against a composition of beeswax and tallow placed in the groove, the surface of which was slightly melted by a heater, carried on each train, in order to secure an air-tight joint. Connected behind the piston was a frame carrying four wheels which lifted and sustained the continuous valve for a distance of about 15 ft. Thus the piston having atmospheric pressure on one side of it and a vacuum equal to 15 or 16 in. of mercury on the other, was forced along the tube, taking the train with it. Various advantages were claimed by the advocates of the system, including cheapness of operation as compared with steam locomotives, and safety from collision, because the main was divided into sections by separating valves and only one train could be in each section at a given time. It was installed on about 2 m. of line between Kingstown and Dalkey (Ireland) in 1843 and worked till 1855; it was also tried on the London and Croydon and on the South Devon lines, but was soon abandoned. The same principle is applied in the system of pneumatic despatch (q.v.) to the transmission of small parcels in connexion with postal and telegraph work.

For further particulars see three papers by J. Samuda, P.W. Barlow and
G. Berkeley, with reports of the discussions upon them, in _Proc.
Inst. C.E._, 1844 and 1845.

ATOLL (native name _atollon_ in the Maldive Islands), a horse-shoe or ring shaped coral reef enclosing a lagoon. The usual shape is that of a partly submerged dish with a broken edge, forming the ring of islands, standing upon a conical pedestal. The dish is formed of coral rock and the shells of various reef-dwelling mollusca, covered, especially at the seaward edges, with a film of living coral polyps that continually extend the fringe, and enlarge the diameter of the atoll. The lagoon tends to deepen when the land is stationary by the death of the coral animals in the still water, and the patchy disintegration of the "hard" coral, while waves and storms tear off blocks of rock and pile them up at the margin, increasing the height of the islands, which become covered by vegetation. The lagoon entrance in the open part of the horse-shoe is always to leeward of prevailing winds, since the coral growth is there slower than where the waves constantly renew the polyps' food supply. The conical pedestal rising from the depths is frequently a submarine volcanic cone or island, though any submerged peak may be crowned by an atoll. For the theory of atoll formation see CORAL-REEFS.

ATOM

Theories of matter.

(Gr. [Greek: atomos], indivisible, from [Greek: a-] privative, and [Greek: temnein], to cut), the term given in physical science to the ultimate indivisible particle of matter, and so by analogy to something minutely small in size. If we examine such a substance as sugar we find that it can be broken up into fine grains, and these again into finer, the finest particles still appearing to be of the same nature as sugar. The same is true in the case of a liquid such as water; it can be divided into drops and these again into smaller drops, or into the finest spray the particles of which are too small to be detected by our unaided vision. In fact, so far as the direct evidence of our senses tells us, matter appears to be indefinitely divisible. Moreover, small particles do not seem to exist in the water until it is broken up; so far as we can see, the material of the water is continuous not granular. This conception of matter, _as infinitely divisible and continuous_, was taught by Anaxagoras more than four centuries before the Christian era, and in the philosophy of Aristotle the same ideas are found. But some phenomena are difficult to reconcile with this view; for example, a cubic foot of air can be compressed into less than one five-hundredth of a cubic foot, or, if allowed to expand, the air originally occupying the cubic foot can be made to fill, apparently uniformly, a space of a million cubic feet or more. This enormous capacity for expansion and contraction is astonishing if we believe matter to be continuous, but if we imagine air to be made up of little particles separated by relatively large empty spaces the changes in volume are more easily conceivable. Moreover, if we attribute such a structure to gases, we are led to attribute it to liquids and to solids also, since gases can be liquefied without any abrupt change, and many substances usually solid can be converted into gases by heating them. This conception of the _grained_ structure of matter is very ancient; traces of it are to be found in Indian philosophy, perhaps twelve centuries before the Christian era, and the Greek philosophers Democritus and Epicurus, in the 3rd and 4th centuries B.C., taught it very definitely. Their view was that "matter is not indefinitely divisible, but that all substances are formed of indivisible particles or atoms which are eternal and unchangeable, that the atoms are separated from one another by void, and that these atoms, by their combinations, form the matter we are conscious of." The Roman poet Lucretius (_De Rerum Natura_) was an eloquent exponent of this theory, but throughout the middle ages, indeed until the 17th century, it was eclipsed by the prestige of Aristotle. In the time, however, of Boyle[1] and Newton, we again find an atomic theory of matter; Newton[2] regarded a gas as consisting of small separate particles which repelled one another, the tendency of a gas to expand being attributed to the supposed repulsion between the particles.

Let us consider some common phenomena in the light of these rival theories as to the nature of matter. When a few lumps of sugar are added to a glass of water and stirred, the sugar soon disappears and we are left with a uniform liquid resembling water, except that it is sweet. What has become of the sugar? Does it still exist? The atomist would say, "Yes, it is broken up into its atoms, and these are distributed throughout the spaces between the particles of water." The rival philosopher, who believes water to be continuous and without spaces between its particles, has a greater difficulty in accounting for the disappearance of the sugar; he would probably say that the sugar, and the water also, had ceased to exist, and that a new continuous substance had been formed from them, but he could offer no picture of how this change had taken place. Or consider a well-marked case of what we are in the habit of calling _chemical combination_. If 127 parts of iodine, which is an almost black solid, and 100 parts of mercury, which is a white liquid metal, be intimately mixed by rubbing them together in a mortar, the two substances wholly disappear, and we obtain instead a brilliant red powder quite unlike the iodine or the mercury; almost the only property that is unchanged is the weight. The question again arises, what has become of the original substances? The atomist has an easy answer; he says that the new body is made up by the juxtaposition of the atoms of iodine and mercury, which still exist in the red powder. His opponent would be disposed to say that the iodine and the mercury ceased to exist when the red powder was formed, that they were _components_ but not _constituents_ of it. The fact that the two components can be recovered from the compound by destroying it does not decide the question. It is remarkable that pure chemistry, even to-day, has no very conclusive arguments for the settlement of this controversy; but the sister science of physics is steadily accumulating evidence in favour of the atomic conception.

[Illustation: From Dalton's _New System of Chemical Philosophy_.]

Hydrogen Gas.
Nitrous Gas.
Carbonic Acid Gas.]

[Illustation:

(.) hydrogen.
( ) oxygen.
(|) nitrogen.
(O) carbon.
(.)( ) water.
(.)(|) ammonia.
(.)(O) ethylene.
(O)( ) carbon monoxide.
( )(O)( ) carbon dioxide.
(|)( ) nitric oxide (nitous gas).
(|)( )(|) nitrous oxide.
( )(|)( ) nitrogen peroxide.]

Dalton.

Until the time of John Dalton, the atomic conception remained purely qualitative, and until then it does not appear to have advanced chemistry or to have found further confirmation in the facts of chemistry. Dalton (1803) gave the atomic theory a quantitative form, and showed that, by means of it, a vast number of the facts of chemistry could be predicted or explained. In fact, he did so much to make the atomic theory of matter probable that he is popularly regarded as its originator. Dalton lived in a period marked by great advances in experimental chemistry. Rather before the commencement of the 19th century the work of Lavoisier had rendered it very probable that chemical changes are not accompanied by any change in weight, and this principle of the conservation of matter was becoming universally accepted; chemists were also acquiring considerable skill in chemical analysis, that is, in the determination of the nature and relative amounts of the elements contained in compounds. But Sir H.E. Roscoe and A. Harden, _New View of the Atomic Theory_ (1896), have shown, from a study of Dalton's manuscript notes, that we do not owe his atomic theory to such experiments. If their view is correct, the theory appears to be a remarkable example of deductive reasoning. Dalton, who was a mathematical physicist even more than a chemist, had given much thought to the study of gases. Following Newton, he believed a gas to be made up of particles or atoms, separated from one another by considerable spaces. Certain difficulties that he met with in his speculations led him to the conclusion that the particles of any one kind of gas, though all of them alike, must differ from those of another gas both in _size_ and _weight_. He thus arrived at the conception of a definite atomic weight peculiar to the particles of each gas, and he thought that he could determine these atomic weights, in terms of one of them, by means of the quantitative analysis of compounds. The conclusion that each element had a definite atomic weight, peculiar to it, was the new idea that made his speculations fruitful, because it allowed of quantitative deduction and verification. He drew simple diagrams, three of which, taken from Dalton's _New System of Chemical Philosophy_, part ii. (1810), are reproduced here, in which gases are represented as composed of atoms. Knowing that the gas which he called "nitrous gas" was composed of oxygen and nitrogen, and believing it to be the simplest compound of these two elements, he naturally represented its atom as formed of an atom of oxygen and an atom of nitrogen in juxtaposition. When two elements form more than one compound, as is the case with oxygen and carbon, he assigned to the compound which he thought the more complex an atom made up of two atoms of the one element and one atom of the other; the diagram for carbonic acid illustrates this, and an extension of the same plan enabled him to represent any compound, however complex its structure. The table here given contains some of Dalton's diagrams of atoms. They are not all considered to be correct at the present time; for example, we now think that the ultimate particle of water is made up of two atoms of hydrogen and one of oxygen, and that that of ammonia contains three atoms of hydrogen to one of nitrogen. But these differences between Dalton's views and our present ones do not impair the accuracy of the arguments which follow. The diagrams show that Dalton formed a very definite conception of the nature of chemical combination; it was the union of a small number of atoms of one kind with a small number of another kind to form a compound atom, or as we now say a "molecule," this identical process being repeated millions of times to form a perceptible amount of a compound. The conceptions of "element," "compound" and "mixture" became more precise than they had been hitherto; in an element all the atoms are alike, in a compound all the molecules are alike, in a mixture there are different kinds of molecules. If we accept the hypothesis that each kind of atom has a specific and invariable weight, we can, with the aid of the above theory, make most important inferences concerning the proportions by weight in which substances combine to form compounds. These inferences are often summarized as the laws of _constant, multiple and reciprocal proportions_.

Law of constant proportions.

The law of _constant proportions_ asserts that _when two elements unite to form a compound the weights that combine are in an invariable ratio, a ratio that is characteristic of that compound._ Thus if Dalton's diagram for the molecule, or compound atom, of water be correct, it follows that in all samples of water the total number of the hydrogen atoms is equal to that of the oxygen atoms; consequently, the ratio of the weight of oxygen to that of hydrogen in water is the same as the ratio of the weights of an oxygen and a hydrogen atom, and _this is invariable_. Different samples of water cannot therefore differ ever so little in percentage composition, and the same must be true for every compound as distinguished from a mixture. Apart from the atomic theory there is no obvious reason why this should be so. We give the name bread to a substance containing variable proportions of flour and water. Similarly the substance we call wine is undeniably variable in composition. Why should not the substance we call water also vary more or less? The Aristotelian would find no difficulty in such a variability; it is only the disciple of Dalton to whom it seems impossible. It is evident that we have in this law a definite prediction that can be tested by experiment.

Law of multiple proportions.

The law of _multiple proportions_ asserts that _if two elements form more than one compound, then the weights of the one element which are found combined with unit weight of the other in the different compounds, must be in the ratio of two or more whole numbers._ If we compare Dalton's diagrams of the two oxides of carbon or of the three oxides of nitrogen that are given in the preceding table, we at once see the necessity of this law; for the more complex molecule has to be formed from the simpler one by the addition of one or more whole atoms. In the oxides of carbon the same weight of carbon must be combined with weights of oxygen that are as 1 : 2, and in the oxides of nitrogen a fixed weight of nitrogen must be in union with weights of oxygen that are as 1 : 2 : 1/2, which are the same ratios as 2 : 4 : 1. This law has been abundantly verified by experiment; for example, five oxides of nitrogen are known, and independent analyses show that, if we consider the same weight of nitrogen in every case, the weights of oxygen combined with it are to one another as 1 : 2 : 3 : 4 : 5. The discovery of this law is due to Dalton; it is a direct deduction from his atomic theory. Here again, apart from this theory, there is no obvious reason why the composition of different substances should be related in so simple a way. As Dalton said, "The doctrine of definite proportions appears mysterious unless we adopt the atomic hypothesis." "It appears like the mystical ratios of Kepler which Newton so happily elucidated." The chemists of Dalton's time were not unanimous in accepting these laws; indeed C.L. Berthollet (_Essai de statique chimique_, 1803) expressly controverted them. He maintained that, under varying conditions, two substances could combine in an indefinitely large number of different ratios, that there could in fact be a continuous variation in the combining ratio. This view is clearly inconsistent with the atomic theory, which requires that when the combining ratio of two substances changes it should do so, _per saltum_, to quite another value.

Law of reciprocal proportions.

The law of _reciprocal proportions_, or, as it might well be named, the law of _equivalence_, cannot be adequately enunciated in a few words. The following gives a partial statement of it. _If we know the weights a and b of two elements that are found in union with unit weight of a third element, then we can predict the composition of the compounds which the first two elements can form with each other; either the weights a and b will combine exactly, or if not, these weights must be multiplied by integers to obtain the composition of a compound._ To see how this law follows from Dalton's theory let us consider his diagrams for the molecules of water, ethylene and the oxides of carbon. In water and in ethylene experiment shows that 8 parts by weight of oxygen and 6 parts of carbon, respectively, are in union with one part of hydrogen; also, if the diagrams are correct, these numbers must be in the ratio of the atomic weights of oxygen and carbon. We can therefore predict that all oxides of carbon will have compositions represented by the ratio of 8m parts of oxygen to 6n parts of carbon, where m and n are whole numbers. This prediction is verified by the result of analysis. Similarly, if we know by experiment the composition of water and of ammonia, we can predict the probable composition of the oxides of nitrogen. Experiment shows that, in water and ammonia, we have, respectively, 8 parts of oxygen and 4.67 parts of nitrogen in union with one part of hydrogen; we can therefore infer that the oxides of nitrogen will all have the composition of 8m parts of oxygen to 4.67n parts of nitrogen. Experiment alone can tell us the values of m and n; all that the theory tells us is that they are whole numbers. In this particular case, n turns out to be 3, and m has in succession the values 1, 2, 3, 4, 5.

It is evident that these laws all follow from the idea that a compound molecule can only alter through the addition or subtraction of one or more complete atoms, together with the idea that all the molecules in a pure substance are alike. Fortunately, the compounds at first examined by the chemists engaged in verifying these laws were comparatively simple, so that the whole numbers referred to above were small. The astonishing variety of ratios in which carbon and hydrogen combine was not at first realized. Otherwise Berthollet's position would have been a much stronger one, and the atomic theory might have had to wait a long while for acceptance. Even at the present time, it would be too much to say that all the complex organic substances have been proved by analysis to obey these laws; all we can assert is that their composition and properties can be satisfactorily explained on the assumption that they do so.

The above statement does not by any means exhaust the possible predictions that can be made from the atomic theory, but it shows how to test the theory. If chemical compounds can be proved by experiment to obey these laws, then the atomic theory acquires a high degree of probability; if they are contradicted by experiment then the atomic theory must be abandoned, or very much modified. Dalton himself made many analyses with the purpose of establishing his views, but his skill as an analyst was not very great. It is in the work of the great Swedish chemist J.J. Berzelius, and somewhat later, in the experiments of the Belgian chemist J.S. Stas, that we find the most brilliant and vigorous verification of these laws, and therefore of the atomic theory.

We shall now give an outline of the experimental evidence for the truth of these laws.

Experimental evidence.

The law of the conservation of matter, an important element in the atomic theory, has been roughly verified by innumerable analyses, in which, a given weight of a substance having been taken, each ingredient in it is isolated and its weight separately determined; the total weight of the ingredients is always found to be very nearly equal to the weight of the original substance. But on account of experimental errors in weighing and measuring, and through loss of material in the transfer of substances from one vessel to another, such analyses are rarely trustworthy to more than one part in about 500; so that small changes in weight consequent on the chemical change could not with certainty be proved or disproved. A few experimenters have carried the verification much further. Stas, in his syntheses of silver iodide, weighed the silver and the iodine separately, and after converting them into the compound he weighed this also. In each of a number of experiments he found that the weight of the silver iodide did not differ by one twenty-thousandth of the whole from the sum of the weights of the silver and the iodine used. His analyses of another compound, silver iodate, confirm the law to one part in 78,000. In E.W. Morley's experiments on the synthesis of water the hydrogen, the oxygen and the water that had been formed were separately determined; taking the mean of his results, the sum of the weights of the ingredients is not found to differ from the weight of the product by one part in 10,000. It is evident that if our experiments are solely directed to the verification of this law, they should, if possible, be carried out in a hermetically closed vessel, the vessel and its contents being weighed before and after the chemical change. The extremely careful experiments of this kind, by H. Landolt and others, made it at first appear that the change in weight, if there is any, consequent on a chemical change can rarely exceed one-millionth of the weight of the reacting substances, and that it must often be much less. The small discrepancies found are so easily accounted for by attributing them to experimental errors that, until recently, every chemist would have regarded the law as sufficiently verified. Landolt's subsequent experiments showed, what was already noticed in the earlier ones, that these minute changes in weight are nearly always losses, the products weigh less than the components, while if they had been purely experimental errors, due to weighing, they might have been expected to be as frequently gains as losses. Landolt was disposed to attribute these losses in weight to the containing vessel, which was of glass or quartz, not being absolutely impervious, but in 1908 he showed that, by making allowance for the moisture adsorbed on the vessel, the errors were both positive and negative, and were less than one in ten million. He concluded that _no change of weight can be detected._ Modern researches (see RADIOACTIVITY) on the complex nature of the atom have a little shaken the belief in the absolute permanence of matter. But it seems pretty clear that if there is any change in weight consequent on chemical change, it is _too minute to be of importance to the chemist_, though the methods of modern physics may settle the question. (See ELEMENT.)

The law of constant proportions is easily verified to a moderate degree of accuracy by such experiments as the following. We can prepare, in the laboratory, a white powder that proves to be calcium carbonate, that is, it appears to be wholly composed of carbon dioxide and lime. We find in nature two other unlike substances, marble and Iceland spar, each of which is wholly composed of carbon dioxide and lime. Thus these three substances, unlike in appearance and origin, are composed of the same ingredients: if small variations in the combining ratio of the components were possible, we might expect to find them in such a case as this. But analysis has failed to find such differences; the ratio of the weights of lime and carbon dioxide is found to be the same in all three substances. Such analyses, which do not always admit of great accuracy, have been confirmed by a few carefully planned experiments in which two components were brought together under very varied conditions, and the resulting compound analysed. Stas carried out such experiments on the composition of silver chloride and of ammonium chloride, but he never found a variation of one part in 10,000 in the composition of the substances.