Pressure:—The weight or pressure of the atmosphere is shown by the rise of water in the barrel of the common ‘lifting pump’ and the suspension of the mercurial column in the tube of the barometer. The last affords a ready means of determining the actual pressure of the air, the column of mercury, and the column of air by which it is suspended, resembling two weights in equilibrio, at the opposite extremities of the same balance. The mean height of the barometer at the level of the sea, in England, is 28·6 inches (= about 331⁄2 feet of water); and as a cubic inch of mercury weighs 3425·92 gr., or ·48956 lb., it follows that the weight of a column of mercury whose base is a square inch is 14·6 lbs. avoirdupois. The pressure of the atmosphere is not merely downwards, but is equally diffused in all directions, and exerts a most powerful effect in the economy of organic beings. On the surface of the body of an adult of ordinary size (say = 15 sq. feet, or 2160 inches), it amounts to the enormous weight of 31,536 lbs., which is not sensible, only because it is balanced by the force of the elastic fluids in the interior of the body. Were this equilibrium to be suddenly destroyed, the consequence would be, either that the body would be instantly torn to pieces with explosive violence, or that it would be crushed under the overwhelming weight that would suddenly fall upon it. Even the comparatively slight variations of atmospheric pressure which occur with changes of wind, weather, and
season, exercise a perceptible effect on the functions of life.
Mean pressure of the Atmosphere at the level of the Sea, in different latitudes, at 32° Fahr., expressed in inches of mercury.
| Lat. | Height (inches). | Lat. | Height (inches). | Lat. | Height (inches). |
| 0° | 29·930 | 40° | 30·019 | 541⁄2° | 29·926 |
| 10 | 29·975 | 45 | 30·000 | 60 | 29·803 |
| 20 | 30·064 | 49 | 29·978 | 64 | 29·606 |
| 30 | 30·108 | 511⁄2 | 29·551 | 67 | 29·673 |
Temperature:—The temperature of the atmosphere, independently of changes arising from variations of latitude and season, diminishes, like its density, with its elevation. In general, every 100 yards of ascent causes the temperature to fall 1° Fahr. See Air (Atmospheric), Epidemics, Ventilation, &c.
Atmosphere. In engineering and pneumatics, the pressure of a column of mercury at 0° Cent. or 32° Fahr., which is 76 centimètres or 29·9218 inches high, at the mean level of the sea in latitude 45°, taken as a standard of that exerted by other elastic fluids. In practice this is assumed to be 15 lbs. to the square inch, under a barometrical pressure of 30 inches. Thus, steam or air condensed so as to exert a pressure of 30 lbs. per sq. inch is said to be of two atmospheres; at 45 lbs., of three atmospheres, &c.
AT′OM (-ŭm). Atomic Weight, Atomic Theory. Syn. At′omus, L.; Atome, Fr.; Atom, Untheilbare THEILCHEN, Ger.
Atomic Weight. When the elements unite chemically, they invariably do so in the proportions by weight represented by the numbers attached to them in the following table, or in multiples of these proportions. Dalton accounted for this law by supposing that the constituent particles of matter are indivisible, and believed that, if it were possible to place such particles in the balance, their relative weights would be found to correspond with the numbers given in the table.[101] In other words, the term atom, which is derived from the Greek ατομος, indivisible, is applied in modern chemistry to the smallest quantity by weight of an element which is capable of existing in a chemical compound, hydrogen being taken as unity.
[101] Strictly speaking, Dalton, the inventor of the Atomic Theory, did not adopt the precise numbers given in the table, but others, which, however, bear a very simple relation to them.
| Name. | Symbol. | Atomic weight. | Atomic volume. |
| ALUMINUM | Al | 27·5 | |
| Antimony | Sb | 122 | |
| Arsenic | As | 75 | 1⁄4 |
| Barium | Ba | 137 | |
| Bismuth | Bi | 208 | |
| Boron | B | 11 | |
| BROMINE | Br | 80 | 1 |
| Cadmium | Cd | 112 | 2 |
| Cæsium | Cs | 133 | |
| CALCIUM | Ca | 40 | |
| CARBON | C | 12 | |
| Cerium | Ce | 92 | |
| CHLORINE | Cl | 35·5 | 1 |
| Chromium | Cr | 52·5 | |
| Cobalt | Co | 58·8 | |
| COPPER | Cu | 63·5 | |
| Didymium | D | 96 | |
| FLUORINE | F | 19 | 1 |
| Glucinum | G | 14 | |
| Gold | Au | 196·7 | |
| HYDROGEN | H | 1 | 1 |
| Indium | In | 74 | |
| IODINE | I | 127 | 1 |
| Iridium | Ir | 198 | |
| IRON | Fe | 56 | |
| Lanthanum | L | 92 | |
| LEAD | Pb | 207 | |
| Lithium | Li | 7 | |
| Magnesium | Mg | 24 | |
| MANGANESE | Mn | 55 | |
| MERCURY | Hg | 200 | 2 |
| Molybdenum | Mo | 92 | |
| Nickel | Ni | 58·8 | |
| Niobium | Nb | 97·6 | |
| NITROGEN | N | 14 | 1 |
| Osmium | Os | 199 | |
| OXYGEN | O | 16 | 1 |
| Palladium | Pd | 106·5 | |
| PHOSPHORUS | P | 31 | 1⁄4 |
| Platinum | Pt | 197·4 | |
| POTASSIUM | K | 39 | |
| Rhodium | Rh | 104 | |
| Rubidium | Rb | 85·5 | |
| Ruthenium | Ru | 104 | |
| Selenium | Se | 79 | 1 |
| SILICON | Si | 28·5 | |
| SILVER | Ag | 108 | |
| SODIUM | Na | 23 | |
| Strontium | Sr | 87·5 | |
| SULPHUR | S | 32 | 1 |
| Tantalum | Ta | 137·5 | |
| Tellurium | Te | 128 | |
| Thallium | Tl | 204 | |
| Thorium | Th | 231·5 | |
| Tin | Sn | 118 | |
| Titanium | Ti | 50 | |
| Tungsten | W | 184 | |
| Uranium | U | 120 | |
| Vanadium | V | 51·2 | |
| Yttrium | Y | 68 | |
| ZINC | Zn | 65 | 2 |
| Zirconium | Zr | 90 |