n. [ NL., fr. L. radius ray. ] (Chem.) An intensely radioactive metallic element found (combined) in minute quantities in pitchblende, and various other uranium minerals. Symbol, Ra; atomic weight, 226.4. Radium was discovered by M. and Mme. Curie, of Paris, who in 1902 separated compounds of it by a tedious process from pitchblende. Its compounds color flames carmine and give a characteristic spectrum. It is divalent, resembling barium chemically. The main isotope of radium found in pitchblende, radium-226, has a half-life of 1620 years, decaying first by alpha emission to radon. Radium preparations are remarkable for maintaining themselves at a higher temperature than their surroundings, and for their radiations, which are of three kinds: alpha rays, beta rays, and gamma rays (see these terms). The beta and gamma rays seen in radium preparations are in fact due to disintegration of decay products of radium rather than the radium itself. By reason of these rays they ionize gases, affect photographic plates, cause sores on the skin, and produce many other striking effects. Their degree of activity depends on the proportion of radium present, but not on its state of chemical combination or on external conditions. The radioactivity of radium is therefore an atomic property, and is due to an inherent instability of the atomic nucleus which causes its decay in a process whose rate is first order. The disintegration of the radium nucleus is only the first in a series of nuclear disintegrations leading to production of a series of elements and isotopes. The chain has at least seven stages; the successive main products have been studied and are radon, a gaseous radioactive element belonging chemically to the inert noble gas series (originally called radium emanation or exradio, radium A, radium B, radium C, etc. The successive products are unstable isotopes of several different elements, each with an atomic weight a little lower than its predecessor. Lead is the stable end product. At the same time, the light gas helium is formed, being generated when the expelled alpha particles (positively charged helium nuclei) acquire electrons. Radium, in turn, is formed in the pitchblende ore by a slow disintegration of uranium. Natural radium and also an isotope (radium-228, also called mesothorium I) formed by the decay of thorium, were at one time used to make a luminous paint for watch dials, until the danger of the radioactivity became fully appreciated, and use of such material in watches was discontinued. See also mesothorium. [ Webster 1913 Suppl. +PJC ]

n.; pl. L. Radii E. Radiuses [ L., a staff, rod, spoke of a wheel, radius, ray. See Ray a divergent line. ] 1. (Geom.) A right line drawn or extending from the center of a circle to the periphery; the semidiameter of a circle or sphere. [ 1913 Webster ]

2. (Anat.) The preaxial bone of the forearm, or brachium, corresponding to the tibia of the hind limb. See Illust. of Artiodactyla. [ 1913 Webster ]

☞ The radius is on the same side of the limb as the thumb, or pollex, and in man it is so articulated that its lower end is capable of partial rotation about the ulna. [ 1913 Webster ]

3. (Bot.) A ray, or outer floret, of the capitulum of such plants as the sunflower and the daisy. See Ray, 2. [ 1913 Webster ]

4. pl. (Zool.) (a) The barbs of a perfect feather. (b) Radiating organs, or color-markings, of the radiates. [ 1913 Webster ]

5. The movable limb of a sextant or other angular instrument. Knight. [ 1913 Webster ]

Radius bar (Mach.), a bar pivoted at one end, about which it swings, and having its other end attached to a piece which it causes to move in a circular arc. --
Radius of curvature. See under Curvature. [ 1913 Webster ]

‖ 1. (Math.) A straight line (or the length of such line) connecting any point, as of a curve, with a fixed point, or pole, round which the straight line turns, and to which it serves to refer the successive points of a curve, in a system of polar coordinates. See Coordinate, n. [ 1913 Webster ]

2. (Astron.) An ideal straight line joining the center of an attracting body with that of a body describing an orbit around it, as a line joining the sun and a planet or comet, or a planet and its satellite. [ 1913 Webster ]