Atoll

a Maldive word meaning a ring-shaped with a central lagoon of calm water, as Whit sunday Island in the Pacitic. Atom. An atom was thought until quite reto be the smallest particle of an element, indivisible. But every atom consists of a of infinitely small corpuscles, named which are in a state of continual activity. are not conceived to be capable of existing but always in combination with at least other atom to form a molecule (q.v.). If the in the molecule are alike, as with oxygen, gold, etc., the substance formed by an agof such molecules is called an element; it is a compound. ‘There can be no atom a compound body, and the term is therefore reto the ultimate particles of the elements. Atomic Theory, the name given to that which regards matter as being built up of particles called atoms, to explain obchemical facts by a igning certain physical to these atoms. ‘The true atomic theory modern chemistry is due to Dalton and is only century old, but has done a very great deal to the science of chemistry, and to procure allies in physics and mathematics. The simply states that matter consists ultimately atoms of different kinds, that atoms combine other atoms of like or unlike kind forming and that matter in bulk, such as our perceive it, consists of exceedingly large of segregated molecules. According to the atomic theory, the atom is the smallest quantity of matter that exist in combination, the molecule is the quantity that can exist alone, and must, consist of at least one atom. Mercury zinc give examples of molecules contain only single atoms; hydrogen, oxygen, nitrogen, most other elements have two atoms; the modifications of oxygen have three; whilst and arsenic po e four atoms to the If in a quantity of matter all the moleare alike, the substance is said to be pure; if we have a mechanical mixture. If the atoms of the molecules are all alike wean elementary substance or element. If, while molecules are mike, they are not. composed of atoms, we have a pure chemical compound. elements as we know them are not infinite in hence the available types of atoms are The properties of all atoms of the same t.c. of the same element, are supposed identical the universe, if under the same con- One of the most important physical of an atom is its ma , from which in- Mendeleéff's periodic law of the elements us to deduce other properties of the sub- The ma of an atom is reckoned in terms the ma of the hydrogen atom, which is the piece known to us at present. The atomic weight an element means therefore the ratio of the of its atom to the weight of the bydrogen atom. weights It laws compounds always water oxygen, a umption water one position number compounds ferent which other. and dioxide, different whose and molecule carbon weight tains hence with that three, five tions quantities regards ) Aluminium Antimony. Arsenic Barium Beryllium- Bismuth Boron Bromine Cadmium. Cesium Calcaum Carbon. Oerium- Chlorine ‘Chromium Cobalt Copper- Didymium Erbium Fluorine Gallium Germanium Gold Holium- Hlydrogen- Indium- Iodine Tridium Iron- Krypton Lanthanum Lead Lithium Magnesium Manganese Mercury will (a) (b) (c) Atomic Theory. teeter er The following is a table of the atomic of the known elements:-— - - Al 27 Molybdenum - Mo 96 - - re 120 Neon - - - - Ne 996 Oa don 40 Nickel - - - Ni 586 - - - As 75 Niobium - - ~ Nb 94. -. Ba 187 Nitrogen. - - N 14 - - Be 9 Osmium ~ - O8 195 - - - Bi 208 Oxygen - - - O 16 2 © 6 BR oD Palladium - Pd 106 - - Br 80 Phosphorus - - P81 - - Cdi2 Platinum - - + Pt 144 - - Os 188 Pota ium - - K 39 - - - Ca 40 Radium - - - Ra258 - + « C 12 Rhodium - «- - Rh104 + - - Ce 140 Rubidium- - - Rb 86 - - - Cl 3854] Ruthenium - - Rul0s%$ « - Cr 52 Samarium- - - Sa 150 e+ Co 58°6 Scandium. -. Se 44 - - - Cu 68 Sclunium - - - Se 79 - - Di 142 Silicon- - - « Si 28 - E 166 Silver - - + Ag 10s - - - F 19 Sodium - - Na 28 - - - Ga 70 Strontium - Nr 's - - Go 72°38 Sulphur - - - 8S 83 - - = + Auld6s Tantalum. - + Ta 182 - « He 4 Tollurium- - - Te 125 - - Ho 1 Thallium - - - Ti 204 - - = In 1184 Thorhun - - - Th2s2 - - - - 1 1265 Tin - - - = « Su118 - Jr 1995 Titanium - - Ti 48 - - - + Foe 56 Tungsten. - W 1836 - - - Kr 8 Uranium - - - U 240.. Ta 188 Vanadium. - - Vo 5L - - - - Ph 206% Ytterbium - - Yb178 - -. Li 7 Yttrium - + - Y¥ 89 - - Mg 24 Xenon- - - - Xel28 - - Mn 55 Zine + - - » Zn th - - - Hg200 Zircon in in - - “r now be secn how the following observed of chemical combination may be explained:— The law of fixity of proportions in chemical states that every definite pure substance po e es the same constitution. Thus always contains cight-ninths its weight of with one-ninth of hydrogen. For on the of the atomic theory, each molecule of contains two atoms of hydrogen united with of oxygen. Hence, since the percentage com of cach molecule is a constant, that of any of molecules will also remain the same, The law of maltiple proportions in chemical states that substances may form dif compounds by uniting in fixed proportions, bear some simple numerical relation to each Thus the ratio of the weights of the carboh oxygen in carbon monoxide are 3: 4, in carbon 3:8. So also nitrogen and oxygen unite in proportions, forming a series of oxides constituents are in the ratios 1, +4, 44, 44, }4. ‘These facts are readily explained. A of carbon monoxide contains one atom of and one atom of oxygen, the ratio of whose is 2. The molecule of carbon dioxide conone atom of carbon with two of oxygen; the ratio of the constituents is 4. Similarly the nitrogen oxides, we are led to the belief two atoms of nitrogen unite with one, two, four, and five atoms of oxygen, forming these different kinds of molecules, whose compos i are therefore closely related to each other. The law of chemical equivalents, chemical which are equal to the same thing as their power of doing chemical work or of 274 Atonement. ( 246 sweet eae forming chemical compounds, are equivalent to each other. One gramme of hydrogen will unite with 35-4 grammes of chlorine or with 8 of oxygen. flence 8 grammes of oxygen are chemically equivalent to 35-4 of chlorine, or two atoms of hydrogen combine with two of chlorine or with one of oxygen; hence two atoms of chlorine are equivalent to one of oxygen, and knowing the respective atomic weights the above numerical relationship may be immediately established. ‘The next two laws given are not directly deducible from experiment, relating as they do to individual molecules, (ad) Avogadro's Law,—-Equal volumes of all gases at the same temperature and pre ure contain the same number of molecules, te. molecules of all gases under the same conditions of temperature and pre ure occupy the same space. [MOLECULE. ] (¢) Dulong and Petit’s Law.—Tnhe atomic weight of an element multiplied by its specific heat is a constant for all elements, known as the atomic heat, These two laws receive full confirmation from the kinetic theory of gases, as advanced by Clausius, Clerk-Maxwell, and other physicists, and afford the most conclusive means of settling the atomic weight of an clement, For an explanation of the system of nomenclature adopted in modern chemistry see CHEMISTRY. Atonement, « “ putting at one” or reconciliation of those who were alienated, properly referring to the work of Christ. in reconciling fallen man with God. Sometimes, however, in recent times the word has been used as if it meant satisfaction or payment for sin. Atrato, 2 river in the United States of Colombia, South America. Rising in a spur of the West Cordilleras, it flows almost due north, and after a course of 200 miles discharges itself by nine mouths into the Gulf of Darien. It is navigable for most of its course, and engineers have proposed to connect it by canal with the 5. Juan, which falls into the Pacific, thus providing a substitute for the now practically abandoned Panama Canal.