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Rh Raumes in neuer analytischer Behandlungsweise, but this contains merely a more systematic and polished rendering of his earlier results. In 1847 he was made professor of physics at Bonn, and from that time his scientific activity took a new and astonishing turn.

His first physical memoir, published in Poggendorfs Annalen (1847), vol. lxxii., contains his great discovery of magnecrystallic action. Then followed a long series of researches, mostly published in the same journal, on the properties of magnetic and diamagnetic bodies, establishing results which are now part and parcel of our magnetic knowledge. In 1858 (Pogg. Ann. vol. ciii.) he published the first of his classical researches on the action of the magnet on the electric discharge in rarefied gases.

Plucker, first by himself and afterwards in conjunction with J. W. Hittorf, made many important discoveries in the spectroscopy of gases. He was the first to use the vacuum tube with the capillary part now called a Geissler's tube, by means of which the luminous intensity of feeble electric discharges was raised sufficiently to allow of spectroscopic investigation. He anticipated R. W. Y. Bunsen and G. Kirchhoff in announcing that the lines of the spectrum were characteristic 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.

Hittorf tells us that Plucker never attained great manual dexterity as an experimenter. He had always, however, very clear conceptions 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 secure from the Sayner Hutte in 1846 the great electromagnet which he turned to such 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.

Induced by the encouragement of his mathematical friends in England, Plucker in 1865 returned to the field in which he first became famous, and adorned it by one more great achievement—the invention of what is now called “line geometry.” His first memoir on the subject was published in the Philosophical Transactions of the Royal Society of London. It became the sourcc of a large literature in which the new science was developed. Plucker 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 22nd of May 1868. The work was so far advanced that his pupil and assistant Felix Klein was able to complete and publish it (see ). Among the very numerous honours bestowed on Plucker by the various scientific societies of Europe was the Copley medal, awarded to him by the Royal Society two years before his death.

PLUM, the English name both for certain kinds of tree and also generally for their fruit. The plum tree belongs to the genus Prunus, natural order Rosaceae. Cultivated plums are supposed to have originated from one or other of the species P. domestica (wild plum) or P. insititia (bullace). 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 distinctions, however, are not maintained with much constancy. PP. [sic] domestica IS a native of Anatolia and the Caucasus, and is considered to be the only species naturalized in Europe. P. insititia 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 Romans cultivated large numbers of plums. The cultivated forms are extremely numerous, some of the groups, such as the greengages, the damsons and the egg plums being very distinct, and sometimes reproducing themselves from seed. The colour of the fruit varies from green to deep purple, the size from that of a small cherry to that of a hen's egg; 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, as it is not particular 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.

Plums are propagated chiefly by budding on stocks of the Mussel, Brussels, St Julien and Pear plums. The damson, wine-sour and other varieties, planted as standards, are generally increased by suckers. For planting against walls, trees which have been trained for two years in the nursery are preferred, but maiden trees can be very successfully introduced, and by liberal treatment may be speedily got to a fruiting state. Any good well-drained loamy soil is suitable for plums, that of medium quality as to lightness being decidedly preferable. Walls with an east or west aspect are generally allowed to them. The horizontal mode of training and the fan or half-fan forms are commonly followed; where there is sufficient height probably the fan system is the best. The shoots should be laid in nearly or quite at full length. The fruit is produced on small spurs on branches at least two years old, and the same spurs continue fruitful for several years. Standard plum trees should be planted 25 ft. apart each way, and dwarfs 15 or 20 ft. The latter are now largely grown for market purposes, being more easily supported when carrying heavy crops, fruiting earlier, and the fruit being gathered more easily from the dwarf bush than from standard trees.

The following is a selection of good varieties of plums, with their times of ripening:—

Diseases.—The Plum is subject to several diseases of fungal origin. A widespread disease known as pocket-plums or bladder plums is due to an ascomycetous fungus, Exoascus pruns, the mycelium of which lives parasitically in the tissues of the host plant, passes into the ovary of the flower and causes the characteristic malformation of the fruit which becomes a deformed, sometimes curved or flattened, wrinkled dry structure, with a hollow occupying the place of the stone; the bladder plums are yellow at first, subsequently dingy red The reproductive spores are borne in sacs (asci) which form a dense layer on the surface, appearing like a bloom in July; they are scattered by the wind and propagate the disease. The only remedy is to cut off and burn the diseased branches.

Plum-leaf blister is caused by Polystigma rubrum, a pyrenomycetous fungus which forms thick fleshy reddish patches on the leaves.