610 REPORT— 1894. 



kind relative to boracite, topaz, and various other minerdls, but it seems to be now- 

 established that most of the electrifications observed by means of his method are 

 really piezo-electric, and are due to strains caused by inequality of temperature in 

 different parts of the cooling crystal. A model has been lately made by Lord 

 Kelvin which gives a perfect mechanical representation of the elasticity, the piezo- 

 electricity, and also the pyro-electricity of a crystal. 



Electrical Methods. — A delightfully simple method of investigating 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 

 particles 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 discrimina- 

 tion 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 S^narmont, and afterwards by Rontgen; the 

 magnetic induction treated of by Faraday, Lord Kelvin, Pliicker, and Tyndall ; 

 the hardness 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, Baumgarten, and Koch ; the distortion of crystals in an electro- 

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



Cheviical Relations. — In the short time I can reasonably ask you to allow me 

 it is clearly impossible to enter upon any discussion of the increase of our know- 

 ledge 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. 



Systematic Mineralogy. — As regards systematic mineralogy an immense amount 

 of progress has been made. The condition of affairs in 1 832 was described by Whewell 

 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 

 laiown, or in which two different analyses taken at random might not lead to 

 different formulie ; and no system of classification which has obtained general accep- 

 tation or is maintained, even by its proposer, to be free from gross anomalies.' Ad 

 idea of the extent of the improvement will be best obtained from a comparison of 

 the first edition of Dana's Treatise, published in 1837, and that treasury of infor- 

 mation, the sixth edition, which appeared in 1892. The names of Miller and 

 Uesclcizeaux 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 doubled, and the rate at which new species 

 are discovered shows as yet no 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, aniphiboles, pyroxenes, scapolites, micas, tourmalines, and 

 garnets. A paper just published by Penfield relative to topaz furnishes an 

 excellent 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 the mutual 

 inclination of the optic axes of topaz is very different in different specimens, and it 

 has been suspected that the variation might depend on the percentage of fluorine. 

 Professor Penfield has carefully determined, not only the fluorine, but also the 

 water yielde 1 in the course of analysis of specimens from different localities, and 

 firds that tliH analytical results are best explained by the hypothesis of an iso- 

 jnorphous replacement of fluorine by hydroxyl ; further, he discovers that the 

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

 replacement. 



