1866.] 



President's Address, 



279 



first empirical generalization of the law applicable to crystals of the rhom- 

 bohedral or pyramidal system, and accordingly to uniaxal crystals, he was 

 led to assimilate a crystal to an assemblage of small ellipsoids, capable of 

 magnetic induction, having for their principal planes the planes of crystal- 

 line symmetry where such exist ; and to apply Poisson's theory. The re- 

 sult of this investigation is contained in an elaborate paper read before the 

 Royal Society in 1857, and published in the Philosophical Transactions for 

 the following year. In this paper Professor Pliicker has deduced from 

 theory, and verified by careful experiments, the mathematical laws which 

 regulate the magnecrystallic action. These laws have not necessarily in- 

 volved in them the somewhat artificial hypothesis respecting the magnetic 

 structure of a crystal from which they were deduced ; and at the close of 

 his memoir Professor Pliicker recognizes the theory of Professor Sir William 

 Thomson, with which he then first became acquainted, as a sound basis on 

 which they might be established. The laws, however, remain identically 

 the same in whichever way they may be derived. 



Another subject to which Professor Pliicker has paid much attention is the 

 curious action of powerful magnets on the luminous electric discharge in glass 

 tubes containing highly rarefied gas. In this case the luminous discharge 

 is found to be concentrated along certain curved lines or surfaces. He has 

 succeeded in obtaining the mathematical definition of these curved lines or 

 surfaces, by a simple application of the known laws of electromagnetic ac- 

 tion, regarding an element of the discharge as the element of an electric 

 current. With regard to the blue negative light, for instance, starting 

 from a point in the negative electrode, he has shown that there are two 

 totally distinct paths, one or other of which, according to circumstances, it 

 may take, going either within the enclosed space along a line of magnetic 

 force, or else along the surface of the glass in what he calls an " epipolic 

 curve," which is the locus of a point in which the inner surface of the ves- 

 sel is touched by the line of magnetic force passing through that point. 



Angstrom appears to have been the first to notice that the spectrum of 

 the electric spark striking between metallic electrodes through air on an- 

 other gas at ordinary pressures is a compound one, consisting of very bright 

 lines varying with the metal, and others, usually less bright, depending only 

 on the gas. Under the circumstances which presented themselves in his 

 experiments, the latter can frequently be but ill observed ; and the diffused 

 light of a rarefied gas in a wide tube is but faint, and does not form very 

 definite spectra. But Pliicker found that by employing tubes which were 

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

 tlie narrow part. These spectra were observed by him with great care, 

 and 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. It further appeared that 

 compound gases of any kind were instantaneously, or almost instantaneously, 



