35S 



NATURE 



[August 9, 1S94 



Pyroilectncily. — The development of opposite electricities at 

 diflerent parts of a crystal during changing temperature (pyro- 

 electricitv) has long been known in the case of tourmaline. We 

 owe to Hankel a long series of investigations of this kind 

 relative to boracite, topaz, and various other minerals, but it 

 seems to be now established that most of the electrifications 

 observed by means of his method are really piezo-elcctric, and 

 are due to strains caused by inequality of temperature in 

 difTerent parts of the cooling crystal. .\ model has been lately 

 made by Lord Kelvin which gives a perfect mechanical repre- 

 sentation of the elasticity, the piezo-electricily, and also the 

 pyrn.eleclricity of a crystal. 



Electrica! Methods. — .\ delightfully simple method of in- 

 vestigating the difference of electrical condition of the parts of 

 a cooling crystal and of making the distribution of electricity 

 visible to the eye has been invented by Kundt. Mixed panicles 

 of (red) minium and (yellow) sulphur are oppositely electrified 

 by their passage through the meshes of a small sieve ; falling 

 on the cooling crystal, each particle adheres to the oppositely 

 electrified region, and the electrical condition of the latter is 

 thus immediately indicated by the colour of the adherent powder. 

 Mr. Miers remarks that this method is practically useful as a 

 means of discrimination even when the crystals are extremely 

 minute. 



Other Physical Characters. — Of other physical characters 

 much studied since the issue of Whewell's Report, I may recall 

 to you more especially the dilatation of crystals on change of 

 temperature, in which the observations of Mitscherlich have 

 been extended by Fizeau and Beckenkamp ; the forms of the 

 isothermal surfaces of crystals, as determined by Scnarmonl, 

 and afterwards by R'mlgen ; the magnetic induction treated of 

 by Faraday, Lord Kelvin, Plucker, and Tyndall ; the hardne-s 

 of crystals for different directions lying in the same faces, 

 by Grailich, Pekarek, and Exner ; the elasticity of crystals, 

 investigated by Neumann, Lord Kelvin, Voigt. Haumgarten, 

 and Koch ; the distortion of crystals in an electro-magnetic 

 field, by Kundt, Rontgen, and MM. Curie. 



Chemical Kelalions. — In the short time I can reasonably ask 

 you to allow me it is clearly impossible to enter upon any dis- 

 cussion of the increase of our knowledge of the chemical 

 relations of minerals, and to treat of the much-investigated 

 subjects isomorphism, polymorphism, and morphotropy, nor 

 can I attempt to give you any idea of the advance which 

 has been made towards a natural classification ; nor must I 

 mention the experiments which have been made relative to the 

 growth of crystals, the etching of their faces, or their directions 

 of easiest solution. 



As regards systematic mineralogy an immense amount of pro- 

 gress has been made. The condition of alVairs in 1832 «as 

 described by Whcwell as follows : — " We have very few minerals 

 of which the chemical constitution is not liable to some dispute ; 

 scarcely a single species of which the rules .and limits are known, 

 or in which two different analyses taken at random might not 

 lead to different formulae ; and no system of classification which 

 has obtained general acceptation or is maintained, even by its 

 proposer, to be free from gro^s anomalies." An idea of the 

 extent of the improvement will be best olitained from a com- 

 parison of the first edition of Dana's Treatise, published in 

 1837, and that treasury of inform.ition, the sixth edition, which 

 appeared in 1892. The names of Miller and Descloizeaux are 

 to be honourably mentioned in connection with this detailed 

 work on species. In the interval of time under consideration 

 the number of well-established species has been more than 

 doubted, and the rale at which new species are discovered shows 

 as yet m sign of diminution. In particular, I may remind you 

 of the work which has been done in the correlation of the 

 members of large groups, like the felspars, amphiboles, 

 pyroxene", JcapolitC!, micas, tourmalines, and garnets. A paper 

 just published by I'enfield relative to topaz furnishes an ex- 

 cellent illustration of the important results which are still to be 

 arrived at from a careful study of a common mineral. It has 

 long been known that ihc mutual inclination of the optic axes of 

 topaz is very different in different specimens, and 11 has been 

 tuspeclcd that the variation might depend on the percentage of 

 fluorine. Prof Pcnfield has carefully determined, not only the 

 fluorine, but also the water yielded in the course of analysis of 

 specimens from different localities, and finds that the analytical 

 results are best explained by Ihc hypothesis of an isomorphous 

 replacement of fluorine by hydroxyl ; further, he dis:overs that 



the magnitude of the angle between the optic axes is a function 

 of the amount of that replacement. 



The successes achieved in the artificial formation of minerals, 

 the advances made in the methods of discrimination of minerals 

 by the blowpipe and micro-chemical reactions, the increase in 

 our knowledge of the modes of alteration of minerals, of their 

 association, of their modes of occurrence, must all be left 

 undiscussed. 



Iiistriimenti. — I may add a word relative to the instrumental 

 appliances which have been placed at the service of the 

 mineralogist since the issue of Whewell's Report. .\s regards 

 goniometers, the provision of two mechanical circular move- 

 ments in perpendicular planes for the easier adjustment of a 

 crysial-edge parallel tothe axis of the instrument, first suggested 

 by Viktor von Lang when assistant at the British Museum, has 

 proved a great convenience and is now in general use. The 

 employment of a collimator with interchangeable signals, of a 

 telescope with interchangeable eyepieces, and the provision of 

 lenses and diaphragms for obtaining images from faces so small 

 as to be invisible to the unassisted eye, would seem to have 

 brought the reflective goniometer, the invention of our dis- 

 tinguished countryman Dr. Wollaslon, to a degree of perfection 

 where further improvement is scarcely to be looked for ; though 

 two crystallographers, Kedorow and Goldschmidt, have recently 

 constructed instruments with an additional telescope and entirely 

 diflerent arrangiments. It may be worthy of remark that, 

 though reflective goniometers are generally made for use with 

 very small specimens, one was constructed for the British 

 Museum some years ago by which it is possible to measure the 

 angles of a valuable crystal without removal of the specimea 

 from a matrix of several pounds' weight. 



The polariscope for use with convergent light, the slauro- 

 scope,the employment of polarised light with the microscope, the 

 adaptation of the microscope for the observation of the inter- 

 ference-figures yielded by extremely minute crystals, the 

 spectroscope in the investigation of selective absorption, have 

 all proved of great service in the advancement of our knowledge 

 of the characteis of minerals. 



Worthy of special mention is that recent addition to our re- 

 sources, the total reflectometer, an instrument by which it is 

 possible to determine with wonderful accuracy the refractive 

 index or indices from observ.ition of the reflected light. The 

 process was long ago suggested by Wollaston ; but it is only 

 within the last few years that forms of instrument have been de- 

 vised by Kohlrausch, Sorel, Liebisch, Pulfrich, and Abbe, 

 which make the method as precise in its results as that which 

 depends on refraction by a prism. In its more refined forms 

 the total reHectometer has been used to test the accuracy of the 

 form of Frcsnel's wave-surface : in the convenient, though less 

 precise, form devised by liertrand, the instrument is useful in | 

 the discrimination of the species of minerals. 



For the measurement of the optic axal angle, when the angle 

 is so large that the rays corresponding to the optic axes are 

 totally rellecled at the surface of the plate and do not emerge 

 into air from the crystal. Prof. W. G. .\dams m.ade the valu- 

 able suggestion that the crystal-plate should be interposed 

 between two hemispheres of glass ; several instruments on this 

 principle have been constructed abroad, and have only been 

 imperfectly satisfactory, but one lately made in this country for 

 the British Museum, under the superintendence of my excellent 

 colleague Mr. Miers, proves to be most eflicient for the intended 

 purpose. Mr. Tutton's apparatus for supplying monochromatic 

 light of any desired wave-length is a noteworthy addition to the 

 instrumental resources of the mineralogist. The meldometcr 

 of July for the more accurate determination of the fusing point 

 of minerals should also be recalled to you. 



In this slight sketch it has been possible to make only the 

 barest mention of some of the more important results whichi 

 have been arrived at since the issue of Whewell's Kcport. You 

 will doubtless think that it must have been possible in the yeari 

 1832 to look forward enthusi.astically to the progress which was 

 about to be made. Hut though Professor Whewell was himseli 

 confident that valuable discoveries would reward the mineralo- 

 gical worker, he was s.adly depressed, and, I think I mayi 

 venture to say, with good reason, by the neglect of mineiralo- 

 gical study in this country. I lis own words are ; "This decided 

 check in the progress of the science has, I think, wilhoul| 

 question, damped the interest with which Mineralogy, as t' 

 branch of Natural Philosophy, has been looked upon in Eng 



NO. 1293, VOL. 50I 



