BADIATION. 640 BADIOACTIVITY. inside a hollow solid, whose walls are at a uni- form temperature, is identical with that whifh a 'black body' at that temperature would emit (with certain simple limitations) ; and so. if a narrow slit is made in the walls of this hollow solid, the radiation which escapes can be studied. To absorb and measure the radiation, it is necessary to coat with some 'black' sub- stance a sensitive thermometer or instrument for measuring changes in tem])erature ; for this purpose a bolometer or radio-micrometer or radiometer is covered with lampblack, which is almost 'black' for most waves. In this way the radiation of a 'black' body characteristic of definite temperatures has been studied; and it is possible to express the results in various formuhe. One of these is called Stefan's law; it states that the total radiation of a 'black body' whose surface is S sq. cm., and whose absolute temperature is T (i.e. 273 + t" C), can be expressed as proportional to ST*. Other laws connect the temperature and the radiation of definite wave-lengths. As the temperature is increa.sed, so is the radiation. If two bodies at different temperatures are put together, the one at the higher temperature emits more energy than it absorbs, while the reverse is true of the other body. In the end their temperature should become equal. For a full discussion of the radiation of a 'black body,' reference should be made to the papers by Lummer and Yien, Reports of Inter- national Congress of Physics, vol. ii. (Paris, 1900), and to recent articles by Paschen in Annalen der Physik. The original memoirs by Kirchhoff, Stewart, and others are given in Brace, "Radiation and Absorption," Scientific Memoir Scries, vol. xv. (New York, 1901). See Heat and Light. RADIATION OF HEAT. See Heat. BADICAL AXIS. See Cibcle. RADICAIilSM ( from radical, from Lat. radi- calis, relating to a root, radical, from radix, root; connected with Gk. ^dSt|, rhadix, root, lidSa/ifos, rhadamnos, branch. Goth, tcaurts, root, OHG. umrz, Ger. Wurz, AS. iryrt, Eng. ^rort, weed). In modern history generally, the temper of mind which is most opposed to the conserva- tive, and aims at constant and progressive re- form of political institutions. The word was first used of a political party in England about 1819 though a Radical Party," marked off by clear distinctions from the Whigs, may be said to have originated in 1709 with the first concerted move- ment to reform and control Parliament by press- ure from without. Oddly enough, the efforts of the first Radicals were directed not against the Crown or the House of Lords, but against the House of Commons. Radicalism in its origin was a middle-class movement : its active men were a limited class of voters who conceived that their rights were being infringed. Their first popular leader was Wilkes. A second period extends from 1789 to 1831. The mighty influence of the French Revolution brought up a new class of .Jacobinical Radicals. Under Thomas Paine and Godwin, significant changes took place in their attitude. The Crown and the House of Lords were attacked; com- plaints of oppressive taxation were heard; and the leaders of the school were most of them men who rejected religious creeds. The first quar- ter of the nineteenth century was a discouraging time for the Radicals, but they were not dis- heartened. This period witnessed the rise of the most profound and sj-steniatic philo.sophy of radicalism that had yet been fornnilated. The scientific or philosophical Radicals now came forward, with Bentham and James Jlill, with Ricardo and Grote and Joseph Hume. They gained a powerful organ when in 1824 they founded the AS'estminster Revieic. But there were also practical reformers, and the party was beginning to be recognized as having a definite existence. The working classes now began to take a greater part in the movement, and the first hints of modern socialism were given by Spence and Owen. The third period opens with the definite agi- tation for reform of Parliamentary representa- tion, which is part of general English history. While the Radicals lent their support to the movement for the passage of the Reform Bill, they regarded its results with disappointment, and tended to become more and more sharply dissociated from the Whigs who had passed it. The hope of further enfranchisement of the peo- ple seemed slight; and the feeling of despair thus engendered gave rise to one of the most impor- tant phases in the history of English radicalism, the Cliartist movement. ' (See Ciiartlsji.) The Anti-Corn Law agitation, though the work of Radicals, especially of their two brilliant leaders, Cobden and Bright, w'as not an essential part of their campaign. Yet the Manchester school of politicians may be called the dominant type of Radicals from say 1840 to 1885, the connecting link between those of the beginning and end of the century. Though from a despised and perse- cuted sect, the Radicals, especially under the strong leadership of Chamberlain, succeeded in becoming the controlling force in the Liberal Party, their triumph is not so com])lete as it appears, and many of the reforms to which strict theoretical Radicals have all along been commit- ted seem as far as ever from accomplishment. On the Continent there is a strong tide of Rad- icalism. France has had a strong Radical Party, under one name or another, ever since the out- break of the Revolution. Its tenets are those of the party wherever it is found — the widest pos- sible liberty for the individual. In addition the French Radicals have been the most persistent opponents of monarchy, and to their eti'orts the success of the revolutions of 1830 and 1848 may be attributed. To-day they are a strong party. In Germany the rise of the Radicals is ot a com- paratively recent date. The Socialists, both in Germany and France, must be considered as close- ly allied to the Radicals. Consult: The Radical Prorframme (London, 188.5) ; Lowell, Parties and Governments of Continental Europe, 2d vol. (Boston, 1897); Kent, The English Radicals (London, 1899). See Political Parties. RADICLE, or RADICAL (in chemistry). See Carbox CoMPorNDS. RADIOACTIVITY. The name given to the property which uranium, thorium, and other Ijodies to be described presently have of sending out certain radiations spontaneously. Imme- diately after the discovery of X-rays hy Profes- sor Rontgen in 1895 many ])hysicists began investigations in order to see whether phosphores- cent bodies in general would not emit rays of the
