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LEFT ATMOSPHERIC PRESSUBE 331 ATOMIC WEIGHTS In the lower strata of the atmosphere, the temperature falls at least a degree for every 852 feet of ascent; hence, even in the tropics, mountains of any con- siderable elevation are snow-capped. The atmosphere appears to us blue, because, absorbing the red and yellow solar rays, it reflects the blue one. There appears to be no atmosphere around the moon; but the case seems different with the sun, Venus, Mars, Jupiter, and Saturn. Figuratively, any pervading intellec- tual, moral, religious, or other influence by which one is surrounded; as in the expression, "He lives in an atmosphere of suspicion." ATMOSPHERIC PRESSURE, the pressure exerted by the atmosphere, not merely downward, but in every direc- tion. It amounts to 14.7 pounds of weight on each square inch, which is often called in round numbers 15. On a square foot it is=2,160 pounds, or nearly a ton. It would act upon our bodies with crushing effect were it not that the pressure, operating in all direc- tions, produces an equilibrium. If any gas or liquid press upon a surface with a force of 15 pounds on a square inch, it is generally described as having a pressure of one atmosphere; if 60 pounds, of four atmospheres; if 120 pounds, of eight atmospheres, and so on. ATOLL, the name applied by geologists and others to any one of the lagoon islands, or annular coral reefs found in the Pacific and the Indian Oceans, the Red Sea, and some other parts of the tropics. An atoll is a ring of coral rock, oval rather than circular in form. On the top of the coral rock, which rises but slightly above the sea-level, is vege- tation of some luxuriance. On the con- vex circumference of the ring is a beach of white sand, exterior to which is a line of breakers. The ring of land, which is less than half a mile across, encircles a lagoon. In the larger atolls there are generally two or three breaks in the ring, affording ship channels into the la- goon; these mark the spots where fresh water, discharged from the old subsid- ing land into the sea, prevented the coral animals, which are marine, from locating themselves or building. ATOM, in '.nental philosophy, a particle of matter so infinitely small that it can- not again be subdivided. In natural philosopHy, one of the ex- ceedingly minute ultimate particles of matter, aggregates of an immense num- ber of which, held in their place by mo- lecular forces, constitute all material bodies. In chemistry, the smallest particle into which an element can be divided. An atom cannot exist in a separate state, but unites with one or more atoms to form a molecule. The atoms of differ- ent elements have definite relative weights fixed and invariable for each, the weight of an atom of hydrogen being regarded as unity. ATOMIC PHILOSOPHY, in mental and natural philosophy, the doctrine of atoms, broached by Leucippus, developed by Democritus and modified by Epicurus. It represented atoms as possessed of gravity and motion, and attributed to their union the formation of all things. ATOMIC THEORY, a <theory, first propounded by John Dalton in his "New System of Chemical Philosophy," pub- lished in 1807. He stated that the atoms of each element were incapable of being subdivided, and each had a definite rel- ative weight, compared with that of hy- drogen as 1 ; that the composition of a definite chemical compound is constant; that if two elements, A and B, are ca- pable of uniting with each other in sev- eral proportions, the quantities of B, which unite with a given quantity of A, usually bear a simple relation to one another. Dalton supposed that one ele- ment replaced another atom for atom, but it has since been found that one atom of an element can replace one or more atoms of another element, ac- cording to their respective atomicities. ATOMIC WEIGHTS, the proportions by weight in which the various elemen- tary substances unite together. It is necessary that one element be selected as the starting-point of the series and an arbitrary sum aflSxed to it, so that there- by all the other elements can have their sums awarded to them, according to the proportional amounts in which they com- bine with each other. The second law mentioned under the atomic theory ex- plains the manner in which this can be done, and how far the numbers are arbi- trary. In all systems of atomic weights in modern use, the atomic weight of hy- drogen is taken as unity, and the atomic weight of the other elements are then fixed, so as to give on the whole the sim- plest and most consistent formula for their compounds. There are two systems of atomic weights at present in use: (1) The old system, which, after much discussion, was adopted about 1845; and (2) the new system, which is, in many respects, a revival of the system of Ber- zelius, and which may be said to have come into general use by scientific chem- ists about 1860.