55o HISTORY OF SCIENCE. 



of porous earthenware. D is a solid cylinder of hard gas-retort carbon, 

 which is placed within the porous cell and surrounded by strong nitric 

 acid. The hydrogen, which in the action of the battery would other- 

 wise cover the carbon, is oxidized at the ex- 

 pense of the nitric acid. Fig. 271 shows in 

 plan the mode in which four such Bunsen's 

 cells may be connected with each other. 



Before the year 1820 many experimental 

 and speculative attempts had been made 

 to identify the causes of magnetic and 

 electric phenomena. The analogies be- 

 tween magnetic and electric attractions and 

 repulsions are, indeed, too obvious not to 

 have suggested the idea of the "fluids" 

 ^^^ being either the same in the two classes 



FIG. 271. ' of phenomena, or at least of their being 



closely related to each other. Among others 



who discussed such questions at the beginning of the present century 

 was CHRISTIAN OERSTED (1777 1851), Professor of Natural Philo- 

 sophy at the University of Copenhagen. In a work published in 1812 

 CErsted adduces proofs of the probable identity of electricity and mag- 

 netism. He also occupied himself in attempts to demonstrate this 

 identity by actual experiments. But the attractions and repulsions of 

 the extremities of Volta's piles were too feeble to admit of experimental 

 demonstration in the same way as the forces acting at the poles of a 

 magnet. Besides, the piles presented the peculiarity that when they 

 were in full activity, that is, when their extremities were connected by 

 a conducting- wire, the electrical attractions and repulsions could no 

 longer be observed. But accident, which operated so fortunately in 

 Galvani's discovery, was equally favourable to CErsted. Yet it should 

 be observed that in these, as in most other so-called accidental dis- 

 coveries, the accident happens with any result only to the man whose 

 mind is already prepared for and occupied with similar truths. It is 

 related that in the winter of 1819 CErsted, while lecturing before his 

 class, was exhibiting the heating effects of Volta's pile on a slender 

 wire, and he noticed that a compass-needle which happened acciden- 

 tally to be upon the lecture-table at a little distance was set in oscilla- 

 tion, apparently at the instant the circuit of the pile was completed, 

 that is, when the connecting-wires were joined so that the metallic 

 communication between the two poles of the pile was complete. When 

 his students had withdrawn, CErsted hastened to repeat and vary the 

 arrangement in which the disturbances of the magnetic needle occurred, 

 and he soon found that when the wire joining the poles of the pile was 

 brought near the compass, the needle was strongly deflected. This 

 was the first step on a new path of exploration which the science of elec- 

 tricity now entered upon, and its progress has from that day to this 



