THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FIFTH SERIES.] 



FEBRUARY 1880. 



XL The Dilatation of Crystals on Change of Temperature. By 

 L. Fletcher, M.A., Fellow of University College, Oxford, 

 Assistant in the Mineralogical Department, British Museum*. 



DURING the last fifty years a considerable amount of 

 energy has been devoted by mathematicians and crystal- 

 lographers to the investigation of the nature of the alteration 

 of a crystal due to change of temperature, and it cannot even 

 yet be said that the state of our knowledge is very satisfactory. 

 It must be hoped that this short paper may serve to once more 

 direct attention to this important and interesting question. 

 Mitscherlich was the first to observe that the angles of crystals 

 may vary with the temperature (Pogg. Ann. vol. i. 1824, 

 vol. x. 1827); and he concluded from his experiments that the 

 alteration is probably related to the axes of optical elasticity. 



F. E. Neumann was the next to take up the question (Pogg. 

 Ann. vol. xxvii. 1833), and assuming, as a deduction from 

 experiment, that the luminiferous aether is in every crystal sym- 

 metrical to three perpendicular planes, and that the symmetry 

 of the aether is superinduced by the symmetry of the arrange- 

 ment of the crystal-molecules, decided that the matter of the 

 crystal must also be symmetrical to these three planes, and 

 that on change of temperature the perpendiculars to these 

 planes must remain permanently fixed in direction in space : 

 to these lines he applied the term thermic axes, and gave for- 

 mulae by the help of which their position could be calculated 

 in the case of an oblique crystal from goniometrical observa- 

 tions at any two temperatures. Employing for gypsum the 

 data supplied by Mitscherlich, he found that, within the limits 

 * Coinmunicated "by the Crystallological Society. 



Phil. Mag. S. 5. Vol. 9. No. 54. Feb. 1880. G 



