pered by many restrictions and but little progress was made. In 1828 Guizot, Rémusat, and others started the Revue française, in imitation of the English reviews; it lived, however, only two years. In 1829 appeared the Revue de Paris, which was issued until 1846. The same year (1829) saw the founding of the Revue des Deux Mondes, by Ségur-Dupeyron and Mauroy; during 1830 it was not published, but in 1831 it reappeared, and ever since has maintained the high reputation which it at once attained. Pierre Leroux and George Sand started the Revue Independante in 1841; it ceased to appear in 1848. Many others, equally short-lived, followed it. Among the later literary periodicals are the Nouvelle Revue (1879—); Le Livre (1880—); the Revue de Paris (1894—); and the Revue Britannique (1825-1901).
Modern German Periodicals. In Germany since the beginning of the eighteenth century development has been mainly in the direction of the learned, and, especially in recent times, of the scientific periodical. An extraordinary number of these have been published, many of them of great value. Literature, however, has also been well represented. Only a few of these journals can be mentioned. One of the earliest and most important was the Neue Zeitungen von gelehrten Sachen, founded by J. G. Krause in 1715 and carried on until 1797. “It was the first attempt to apply the form of the weekly political journal to learned subjects.” Still more notable is the Göttingische gelehrte Anzeigen, founded 1739 as the Zeitungen von gelehrten Sachen, and conducted from that time until the present by members of the faculty of the University of Göttingen, among whom have been Haller, Heyne, and Eichhorn. In 1766 the publisher Nicolai founded the Allgemeine deutsche Bibliothek, which was issued until 1806. Lessing and Mendelssohn aided in conducting the Briefe die neueste Litteratur betreffend (1759-65) , also founded by Nicolai. Wieland founded Der deutsche Merkur (1773-89; revived 1790-1810). The Allgemeine Litteraturzeitung, established by Bertuch in 1785 and issued until 1848, was one of the most important of German literary periodicals; the same may be said of the Jenaische allgemeine Litteraturzeitung (1804-48), founded by Eichstädt. The Wiener Jahrbücher der Litteratur (1818-48) enjoyed a high reputation, as did, for its learning, Hermes (1819-31), founded by W. T. Krug. The Jahrbücher für wissenschaftliche Kritik (1827-46), published by Cotta, and the Heidelberger Jahrbücher der Litteratur (1808) should also be mentioned. Of more recent date are the Deutsche Vierteljahrsschrift (1838-70); Die Grenzboten (1841—); Unsere Zeit (1857—); Preussische Jahrbücher (1858—); Die Gegenwart (1872—); the Litteraturzeitung (Jena, 1874—); Deutsche Rundschau (1874—); Die Neue Zeit (1872—); Nord und Süd (1878—); Die Nation 1888—); Die Zukunft (1892—).
Periodicals in Other Countries. The development in other European countries has been similar, though less extensive. Italy, in which, as was stated above, the history of periodical literature dates back to the seventeenth century, exhibits a long list of notable literary journals. Among them are the Frusta letteraria (1763-65) of Giuseppe Baretti; the Novelle letterarie (1740-70) of Giovanni Lami; the Biblioteca italiana (Milan, 1816-40); the Progresso delle scienze (Naples, 1832-45); the Rivista contemporanea (Turin, 1853—); the Giornale degli eruditi (1883—); the Rivista internazionale (1869-83); and the Giornale storico della letteratura italiana (1883—). Spain and Portugal, Belgium, Holland, the Scandinavian countries, Russia, Greece, and the Slavic countries, are all represented by literary periodicals of prominence.
In all the countries mentioned above, periodicals dealing with theology or the interests and practical work of the various religious denominations; with science, either in general or in one or more of its special branches; with the arts or trades, or with other special themes (including periodicals of humor), multiplied greatly during the nineteenth century. Their history cannot be given here even in the briefest summary.
PERIODIC FUNCTION. See Function.
PERIODIC LAW. The generally accepted embodiment of the relations existing between the various properties of the chemical elements, so far as they can be compared with one another. It may be stated as follows: If the elements are arranged in the order of their atomic weights, each of their properties varies as a periodic function of the atomic weight.
Ever since the work of Richter, Proust, and Dalton had established the idea of fixed numerical values attaching to the ingredients of compounds, an idea which was deduced by Dalton from the hypothetical existence of individual atoms, identical in size, mass and other properties for any one element, chemists sought to deduce a closer relationship between the various elements from a comparison of the masses of their respective atoms. (See Chemistry and Atomic Weights.) The first attempt was that made by Dr. Prout in 1815 to prove that all the atomic weights were even multiples of the atomic weight of hydrogen, and that the latter was the only primitive element, from which the others were derived by processes of condensation. It was soon found that very few elements possessed atomic weights that could be expressed by integers, when the atomic weight of hydrogen was set at unity, and Prout's law was gradually modified to state that one-half the atomic weight of hydrogen, then that one-quarter, should be taken as the real standard. Refinements of investigation have since established the relative atomic weights to the second place of decimals, and it can now be asserted that the number of exact coincidences with Prout's law, as compared with that of deviations from it, is not much greater than what would be expected by the theory of chances. Prout's law has, therefore, been practically abandoned. On the other hand, interesting relations were found to exist between the atomic weights of similar elements. Thus Doebereiner established, in 1829, his so-called triads, sets of three closely related elements whose atomic weights were approximately in arithmetical progression, as lithium (7), sodium (23), and potassium (39); calcium (40), strontium (88), and barium (136); sulphur (32), selenium (79), and tellurium (127); chlorine (35.5), bromine (80), and iodine (127); iron (56), nickel (57), and cobalt (58). These triads were later extended to include longer sets, and it was also pointed out that the constant differences were in many cases multiples of 16, the atomic weight of oxygen, whence it was assumed that the heavier elements of a group might
