latter part of the eighteenth century, consists practically of oxygen and
nitrogen in the almost constant proportion of 20.81 volumes of oxygen to
79.19 volumes of nitrogen, or, by weight, 23.01 parts of oxygen to 76.99 of
nitrogen. The gases are associated together, not as a chemical compound,
but as a mechanical mixture. Upon the oxygen present depends the power of
the atmosphere to support combustion and respiration, the nitrogen acting
as a diluent to prevent its too energetic action. It had long been known
that atmospheric nitrogen appeared to have a very slightly greater density
than nitrogen obtained from other sources. Lord Rayleigh and Sir William
Ramsay found that the fact was due to a still more inert gas which forms
nearly 1 per cent of the air, and which had not previously been separated
from nitrogen. This has been named _argon_. Besides these gases, the
atmosphere also contains aqueous vapour in variable quantity, ozone,
carbonic acid gas, traces of ammonia, nitric acid, and, in towns,
sulphuretted hydrogen and sulphurous acid gas. In addition to its gaseous
constituents the atmosphere is charged with dust, bacteria, &c. For other
gases which are present in traces, see _Neon_. See _Climate_;
_Meteorology_.--BIBLIOGRAPHY: C. Flammarion, _L'Atmosphere_; Sir Napier
Shaw, _The Weather Map_.
ATMOSPHERIC ENGINE, name given by early inventors to engines in which the
piston is restored to the bottom of its stroke by atmospheric pressure.
ATMOSPHERIC RAILWAY, so called in consequence of the motive power being
derived from the pressure of the atmosphere, or from compressed air. The
idea of thus obtaining motion was first suggested by the French engineer
Papin, about 200 years ago. In 1810, and again in 1827, Medhurst published
a scheme for 'propelling carriages through a close-fitting air-tight tunnel
by forcing in air behind them'; and in 1825 a similar project was patented
by Vallance of Brighton. About 1835 H. Pinkus, an American residing in
England, patented a pneumatic railway. The carriages were to travel on an
open line of rails, along which a cast-iron tube of between 3 and 4 feet
diameter was to be laid, having a longitudinal slit from 1 to 2 inches wide
and closed by a flexible valve along its upper side, through which a
connection could be formed between the leading carriage and a piston
working within the tube. This method was improved by Messrs. Clegg &
Samuda, who in 1840 tried some experiments on a portion of the West London
Railway with sufficient success to induce the Government to advance a loan
to the Dublin and Kingstown Railway Company, for the construction of a
pneumatic line from Kingstown to Dalkey. It was opened for passenger
traffic at the end of 1843, and was worked for many months. The London and
Croydon Company subsequently obtained powers for laying down an atmospheric
railway by the side of their other line from London to Croydon, and in
experimental trips in 1845 a speed of 30 miles an hour was obtained with
sixteen carriages, and of 70 miles with six carriages. But during the
intense heat of the summer of 1846 the iron tube frequently became so hot
as to melt the composition which sealed the valve, and the line had to be
worked by locomotives. The mechanical difficulty of commanding a sufficient
amount of rarefaction led to the abandonment of the system for railway
purposes. It has been revived, however, for the conveyance of letters and
parcels in towns by means of tubes of moderate diameter laid beneath the
streets. See _Pneumatic Dispatch_.
[Illustration: Atoll]
ATOLL', the Polynesian name for coral islands of the ringed type enclosing
a lagoon in the centre. They are found chiefly in the Pacific in
archipelagos, and occasionally are of large size. Suadiva Atoll is 44 miles
by 34; Rimsky 54 by 20. See _Coral_.
ATOMIC THEORY, a theory as to the existence and properties of atoms (see
_Atoms_); especially, in chemistry, the theory accounting for the fact that
in compound bodies the elements combine in certain constant proportions, by
assuming that all bodies are composed of ultimate atoms, the weight of
which is different in different kinds of matter. It is associated with the
name of Dalton, who systematized and extended the imperfect results of his
predecessors. On its practical side the atomic theory asserts three _Laws
of Combining Proportions_: (1) The Law of Constant or Definite Proportions,
teaching that in every chemical compound the nature and relative weights of
the constituent elements are definite and invariable; thus water invariably
consists of 8 parts by weight of oxygen to 1 part by weight of hydrogen;
(2) The Law of Multiple Proportions, according to which the several
proportions in which one element unites with a given weight of another
invariably bear towards each other a simple relation; thus 1 part by weight
of hydrogen unites with 8 parts by weight of oxygen to form water, and with
16 (i.e. 8x2) parts of oxygen to form peroxide of hydrogen; (3) The Law of
Combination in Reciprocal Proportions, that the proportions in which two
elements combine with a third also represent the proportions in which, or
in some simple multiple of which, they will themselves combine; thus in
olefiant gas hydrogen is present with carbon in the proportion of 1 to 6,
and in carbonic oxide, oxygen is present with carbon in the proportion of 8
to 6, 1 to 8 being also the proportions in which hydrogen and oxygen
combine with each other. The theory that these _proportional numbers_ are,
in fact, nothing else but the relative weights of atoms so far accounts for
the phenomena that the existence of these laws might have been predicted by
the aid of the atomic hypothesis long before they were actually discovered
by analysis. In themselves, however, the laws do not prove the theory of
the existence of ultimate particles of matter of a certain relative weight;
and although many chemists, even without expressly adopting the atomic
theory itself, have followed Dalton in the use of the terms _atom_ and
_atomic weight_, in preference to _proportion_, _combining weight_,
_equivalent_, and the like, yet in using the word _atom_ it should be held
in mind that it merely denotes the combining weights of the elements. These
will remain the same whether the atomic hypothesis which suggested the
employment of the term be true or false. Dalton supposed that the atoms are
spherical, and invented certain symbols to represent the mode in which he
conceived they might combine. The latest atomic hypothesis is one which
assigns an electrical structure to the atom. See _Chemistry_;
_Electricity_; _Matter_. Cf. H. E. Roscoe and A. Harden, _New View of
Dalton's Atomic Theory_; Sir J. J. Thomson, _Atomic Theory_.
ATOMIC WEIGHTS. See _Chemistry_; _Molecular Weights_.
ATOMISTS. See _Atoms_.
ATOMS, for many years regarded as the ultimate indivisible particles of the
chemical elements. The idea originated with some of the ancient
philosophers (the atomists), more especially Democritus (450 B.C.),
Epicurus, and Lucretius (99-55 B.C.), and was developed into a definite
theory by Dalton (1804). According to Dalton the atoms of any one element
are alike in all their properties, but differ from the atoms of other
elements, and when chemical combination occurs it takes place between the
atoms of the combining elements (see _Chemistry_). Various views have been
held with regard to the nature of atoms. Newton regarded them as hard,
ponderable particles, perfectly unalterable, and concluded that the
difference between substances was due to different kinds of atoms. Lord
Kelvin propounded the view that the properties of atoms might be explained
by those of vortices or vortex rings in a homogeneous frictionless fluid.
As a result of the researches of British and French physicists on radium,
the latest view is that matter and electricity are closely connected, that
atoms are not indivisible, but complex aggregates containing positive and
negative electrons, the differences between the atoms depending mainly on
the numbers of these electrons and their velocity. See _Radium_.
ATONEMENT, in Christian theology, the special work of Christ effected by
His life, sufferings, and death. The first explicit exposition of the
evangelical doctrine of the atonement is ascribed to Anselm, Archbishop of
Canterbury, in 1093.
ATRATO ([.a]-trae't[=o]), a river of S. America, in the north-west of
Colombia, emptying itself by nine mouths into the Gulf of Darien; it is
navigable by steamers of some size for 250 miles, and has long been the
subject of schemes for establishing water-communication between the
Atlantic and Pacific.
ATRAULI, a town of India, United Provinces, Aligarh district, clean, well
built, and with a good trade. Pop. 16,560.
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Corrections made to printed original.
Article Animalcule. "but most of them are strictly microscopic.":
'miscroscopic' in original.