BITNSEN MEMORIAL LECTURE. 617 



de Pharm., 1854 (26), 311; 1855 (28), 155), the isolation of whi,-h had 

 hitherto eluded pursuit, and this work he handed over to oui- country- 

 man, Augustus Matthiessen, who, under Hunsen\s g-uidancc, brought 

 the investigation to a successful issue. The conditions most favorable 

 to a reduction were carefully worked out. It had alroadv been pointed 

 out by Bunsen '^that the density of the current (that is, the current 

 per unit cross section) is the chief condition under which the elec- 

 tricity is able to overcome chemical affinities." This condition was 

 fulfilled by using for the negative pole a very short length of thin 

 harpsichord wire, upon which the reduced metal hangs in the form of 

 molten beads, from which they can be quickly detached and plunged 

 into petroleum. Another necessary condition is that the melting point 

 of the electrolyte should be as low as possible, and this was attained 

 by using a mixture of tlie chlorides of calcium and strontium, and l)y 

 the addition of some sal ammoniac to the mass as the electrolvsis 

 proceeds. This subject was again further elaborated, in 1875. in the 

 Heidelberg laboratory, by Hillebrand and Norton (Pogg. Ami.. 1875 

 (156), 466), who prepared considerable quantities of cerium, lanthamim, 

 and didymium in the coherent metallic state. 



But the reduction of the metals was not the only important woi-k 

 which Bunsen got out of his battery, for quite early in its history it 

 made its mark in organic chemistr3^ If by the electrol3"sis of caustic 

 soda we obtain oxygen and a metallic radical, might not the electro- 

 lysis of an organic substance yield the corresponding organic radical ( 

 Doubtless a question of this kind presented itself to Bunsen's mind 

 when he set his assistant, Kolbe, to work on the electrolysis of acetic 

 and valeric acids (Annalen, 1847 (64), 339; 1849 (69), 257). The 

 results of investigations thus commenced and carried out, both by 

 Kolbe alone and in collaboration with Frankland, and the still more 

 prolific researches of the latter chemist, are matters of scientific his- 

 tory; it is, however, not so generally recognized that they owe their 

 origin to the Bunsen battery. 



In addition to the zinc-carbon-nitric acid liattery. Bunsen also con- 

 structed a powerful thermopile of copper pyrites and copper (Pogg. 

 Ann., 1864 (123), 505); and in later years a constant /inc-caibon- 

 chromic acid battery (Pogg. Ann., 1875 (155), 232) so arranged that 

 the zinc and carbon plates could be lowered readily into the exciting 

 liquid, and thus the battery was easily put in and out of action. Tiiis 

 he used for obtaining the spark spectra of the rare earth metals. 



For the purpose of measuring the intensity of the light given oil 

 by carbon poles connected with his battery, Bunsen, in 1S44, con 

 structed his well-known photometer (Berzelius Jahresber.. 1845(24), 

 13). The essential feature of this apparatus is the "disk" of pai)er 

 having a greasedspot in the center, or having a greased <-ircumfer(>nc." 

 with an untouched spot in the middle. With regard to this. Klstcr 



