230 P L U P L U by means of which the luminous intensity of feeble electric discharges was raised sufficiently to allow of spectroscopic investigation. He anticipated Bunsen and Kirehhoff in announcing that the lines of the spectrum were character istic of the chemical substance which emitted them, and in indicating the value of this discovery in chemical analysis. According to Hittorf he was the first who saw the three lines of the hydrogen spectrum, which a few months after his death were recognized in the spectrum of the solar protuberances, and thus solved one of the mysteries of modern astronomy. For a fuller account of the important discoveries regarding the influence of tempera ture and pressure on the nature of gaseous spectra made in conjunction with Hittorf see SPECTRUM ANALYSIS. Hittorf, who had good means of knowing, tells us that Pliicker never attained great manual dexterity as an experimenter. He had always, however, very clear con ceptions as to what was wanted, and possessed in a high degree the power of putting others in possession of his ideas and rendering them enthusiastic in carrying them into practice. Thus he was able to procure from the Sayner Hiitte in 1846 the great electromagnet which he turned to such noble use in his magnetic researches; thus he attached to his service his former pupil the skilful mechanic Fessel ; and thus he discovered and fully availed himself of the ability of the great glass-blower Geissler, in conjunction with whom he devised many of those physical instruments whose use all over the civilized world has rendered the name of the artificer of Bonn immortal. It was thus also that, when he felt his own want of chemical knowledge and manipulative skill, he sought and obtained the assistance of Hittorf, one of the ablest of German experimenters. Induced by the encouragement of his mathematical friends in England, Pliicker in 1865 returned once more to the field in which he first became known to fame, and adorned it by one more great achievement the invention of what is now called Line Geometry. A remark containing the fundamentally new idea of this new geometry had, as Clebsch remarks, already been embodied in the System der Geometric des Raumes : " A straight line depends on four linear constants. The four magnitudes which we consider as variables receive for any given line constant values, which may be easily constructed and are the four coordin ates of the straight line. A single equation between these four coordinates does not determine a locus for the straight line, but merely a law according to which infinite space is made up of straight lines." Here we have the new idea of the straight line considered as an element of space, and of the "complex," as Pliicker afterwards called it, made up of a threefold infinity of straight lines subject to a onefold relation. Space thus becomes as it were four- dimensioned, and we have, instead of the three degrees of freedom of space considered as an aggregate of points, four degrees of freedom according as the linear element is (1) absolutely unconditioned, (2) subject to a onefold, (3) subject to a twofold, or (4) subject to a threefold relation. In the first case we have the complex of straight lines, in the second the congruency of lines, in the third the regulus or ruled surface. The last of these geometri cal figures had been considered long before, and even the congruency had been discussed before or independently of Pliicker, uotably by Hamilton and Rummer. The general conception of the linear complex seems to be entirely due to Pliicker. At all events he developed the notion to such an extent that he is entitled to be called the founder of Line Geometry, in which the theory of the complex holds a fundamental position. His first memoir on the subject was published in the Philosophical Transactions of the Royal Society of London. It attracted much attention, ! and almost at once became the source of a large literature in which the new science was developed. Pliicker himself worked out the theory of complexes of the first and second order, introducing in his investigation of the latter the famous complex surfaces of which he caused those models to be constructed which are now so well known to the student of the higher mathematics. He was engaged in bringing out a large work embodying the results of his researches in Line Geometry when he died on the 22d May 1868. The work was so far advanced that his pupil and assistant Klein was able to complete and publish it, there by erecting the worthiest monument to the genius of his master, whose wonderful scientific activity endured to the very last. Of the very numerous honours bestowed on Pliicker by the various scientific societies of Europe it may suffice to mention here the Copley medal, awarded to him by the Koyal Society two years before his death. Most of the facts in the above notice have been taken from Clebsch s obituary notice of Pliicker (Abh. d. Kon. Ges. d. IViss. z. Gottingen, xvi. , 1871), to which is appended an appreciation of Pliicker s physical researches by Hittorf, and a list of Pliicker s works by F. Klein. See also Gerhardt, Gcschichte der Mathe- matik in Deutschland, p. 282 j and Pliicker s life by Dronke (Bonn, 1871). (6. CH.) PLUM (Prunus). Our cultivated plums are supposed to have originated from one or other of the species P. domestica or P. insititia. The young shoots of P. domestica are glabrous, and the fruit oblong ; in P. insititia the young shoots are pubescent, and the fruit more or less globose. A third species, the common sloe or blackthorn, P. spinosa, has stout spines; its flowers expand before the leaves ; and its fruit is very rough to the taste, in which particulars it differs from the two preceding. These dis tinctions, however, are not maintained with much con stancy. P. domestica is a native of Anatolia and the Caucasus, and is considered to be only naturalized in Europe. P. insititia, on the other hand, is wild in southern Europe, in Armenia, and along the shores of the Caspian. In the Swiss lake-dwellings stones of the P. insititia as well as of P. spinosa have been found, but not those of P. domestica. Nevertheless, the Eomans cultivated large numbers of plums. The cultivated forms are now ex tremely numerous, some of the groups, such as the green gages, the damsons, and the egg plums being very distinct, and even reproducing themselves from seed. This, how ever, cannot be depended on, and hence the choice varie ties are propagated by budding or by layers. The colour of the fruit varies from green, pale yellow, red, up to deep purple, the size from that of a small cherry to that of a walnut; the form is oblong acute or obtuse at both ends or globular ; the stones or kernels vary in like manner ; and the flavour, season of ripening, and duration are all subject to variation. From its hardihood the plum is one of the most valuable fruit trees for the farmer, as it is not parti cular as to soil, and the crop is less likely to be destroyed by spring frosts. Prunes and French plums are merely plums dried in the sun. Their preparation is carried on on a large scale in Bosnia and Servia, as well as in Spain, Portugal, and southern France. The cherry plum, Prvnus myrobalana, is employed chiefly as a stock for grafting upon, and for the sake of its ornamental flowers. See also HORTICULTURE, vol. xii. p. 275. PLUMBAGO, a name frequently applied to graphite in allusion to its remote resemblance to lead, whence it is popularly called "black lead." When Scheele, in 1779, j examined this mineral he regarded it as a compound of carbon and iron, and consequently termed it a "carburet of iron"; but Vanuxem in 1825 showed that the iron existed in the form of an oxide, and was not essential to the constitution of the mineral a conclusion also reached
about this time by Karsten. It thus became fully estab-