lxxxi 



Julius Plttcker, Foreign Member of the Royal Society, was born on 

 the 16th of July 1801, at Elberfeld. After studying in the Gymnasium 

 of Diisseldorf, and in the Universities of Bonn, Berlin, and Heidelberg, he 

 passed some years in Paris. In 1825 he became a Privatdocent of Mathe- 

 matics in Bonn, and in October 1828 was appointed Professor extraordi- 

 narius in that University. In 1833 he went to Berlin in the same capacity, 

 and lectured also in the Friedrich-Wilhelm's Gymnasium. In 1834 he 

 obtained the Professorship of Mathematics in the University of Halle, and 

 in 1836 he was appointed Professor of Mathematics in the University 

 of Bonn. The treatises and memoirs on Analytical Geometry written 

 by him during the twenty years that followed his return from Paris 

 secured for him a place among the first mathematicians of his time. He 

 now entered upon a new career ; for the superintendence of the Physical 

 Museum having been entrusted to his care, he turned his attention to 

 experimental research, and was appointed to the Professorship of Physics 

 in 1847. A series of brilliant discoveries soon placed him among the 

 foremost labourers in this department of science. These researches occu- 

 pied him till 1856. 



In repeating some of Faraday's experiments, he was led to the discovery 

 of magnecrystallic action, — that is, that a crystallized body behaves dif- 

 ferently in the magnetic field according to the orientation of certain di- 

 rections in the crystal. These researches occupied him till 1856, when he 

 turned his attention to the action of powerful magnets on the luminous 

 electric discharge in glass tubes containing highly rarefied gas. In a wide 

 tube the light of such a gas is too faint to permit a satisfactory observa- 

 tion of its spectrum ; he found, however, that by employing tubes which 

 were capillary in one part, brilliant light and definite spectra were obtained 

 in the narrow part. These spectra were found to be characteristic of the 

 several gases and to indicate their chemical nature, though the gases 

 might be present in such minute quantity as utterly to elude chemical 

 research. 



In continuing these researches he next made the remarkable discovery 

 of the two totally different spectra of each of the elementary substances, 

 nitrogen, sulphur, selenium, hydrogen, iodine, lead, manganese, and copper, 

 according as it is submitted to the instantaneous discharge of a Leyden jar 

 charged by an induction coil, or rendered incandescent by the simple dis- 

 charge of the coil, or else, in some cases, by ordinary flames. The two 

 spectra were found to exhibit a difference in character, and are not merely 

 different in the number and position of the lines which they show. This 

 difference he attributed, with the greatest probability, to a difference in 

 the temperature of the gas when the two are respectively produced. These 

 results were made known in a memoir by himself conjointly with Dr. S. W. 

 Hittorf, printed in the Philosophical Transactions for 1865. About this 



vol. xvii. g 



