;54 



NA TURE 



[August q, 1S94 



may appear to you preiamptuou? on my part if I myself make 

 any apilo^y at all, anJ it would doubtless imply a claim to the 

 highest distinction if I were to make that humble apology which 

 would be most appropriate to the circumstances of the case. 



Instead, howiver, of dispensing with the apalogy altogether — 

 that might be too radicil an innovation to be introduced this 

 year — I propose, with your sanction, to make a lesser change, 

 and merely to defer the apology from the first to the last day of 

 our session. I may reasonably hope to be able, at that later 

 stage, to make clear to you, by simple reference to your own 

 experience during the meeting, that any apology I may feel it 

 to be then my duty to make is of no merely formal character, 

 but one which is worthy of your serious consideration. 



I would ask that in the meantime your continuous sympathy 

 be extended to one who now finds himself in a position he 

 would have been the last to seek, and whose ordinary duties in 

 life involve speechless communion with inanimate nature rather 

 than oral address to an assembly of fellow-workers. 



This matter of apologetic precedent being thus disposed of to 

 our common satisfaction, I should have preferred to have 

 brought the delay of the normal business of the Section to an 

 immediate end by calling upon the author of the first paper to 

 now address you. Such, indeed, was the ordinary course of 

 procedure in the earlier, and perhaps presidentially happier, 

 years of the Association ; but the occasion of Inking the chair 

 having been once seized upon, in absence of mind, by a mathe- 

 matical president fir the delivery of an address, it has come 

 abjut that each president now feels it his bounden duty, not 

 merely to give an address, but to make the address at least as 

 long and at least as elaborate as any which has preceded it. 



We shall all agree that a presidential address, if there is to 

 be any at all, should be elaborately short and elaborately simple ; 

 it should deal, not with technical details such as are only intel- 

 ligible, even to the president himself, after much study, but 

 with general principles such as can be immediately grasped by 

 every member of an audience ; an opening address which is so 

 long that it can be only partly read, and is written to be studied 

 afterwards in the Reports of the Association, may more appro- 

 priately be issued as an ordinary memoir. I make this remark 

 to safeguard the interests of future audiences, for the example 

 of technicality which I am now about to set is one which I can- 

 not recommend my successors to follow. 



As for subject, an account of the progress of scientific work 

 is always interesting and instructive, and immediately suggests 

 itself as the natural basis of a presidential address. Uut seeing 

 that, so lately as in February last, the geologists have had the 

 advantage of an address from the retiring president of their 

 Society, .Mr. Iludleston, which has been virtually exhaustive in 

 its survey and criticism of the liritish geological work of the 

 last seven years, the lime has scarcely yet arrived when a con- 

 tinuation of that review by the president of this Section can be 

 of service to the members of the Association. 



For this and oiher still more weighty reasons which I need 

 not directly mention, 1 feel myself debarred from undertaking 

 any review of recent geological progress, and shall therefore ask 

 you to allow me to confine myself, in the remarks it is my duly 

 to make, to a science which, though it is not purely geological 

 and in the Reports of the Association has long been associated 

 with another science, chemistry, is yet very closely related to the 

 science of our own Section, Geology. 



I trust that the members of the Section of Chemistry and 

 Mineralogy are now so clostly engaged in another place that 

 they will tail to discover, or at any rate to resent, the technical 

 trespass on their own domain : as lor yourselves, you will per- 

 haps be more ready to pardon the temporary excursion from the 

 domain of pure geology if I remind you that the fathers of the 

 Geological Society denned their sole object to be " the investi- 

 gation of the mineral structure o( the earth " ; and 1 may add, 

 II further defence be desired, that in the first half of this century 

 the relationship of mineralogy and geology was so intimate that 

 It Wis possible for a Section of the liritish Muicuni to lie 

 officially designated " the Department of Mineralogy, including 

 Geology. " 



1 was the more impelled to choose this subject for our con- 

 sideration to-day when I reflected that pure mineralogy has been 

 hitherto almost completely out of sight, and therefore probably 

 out of mind, at the meetings of the Association. It is true 

 that at the first meeting, held sixty-three years .igo, iJr. 

 Whewell, then the I'rofessor of Mineralogy at Cambridge, was 

 invited to draw up a report on the stale of knowledge of the 



NO. 1293, VOL. 50] 



science, and thai his report was submitted an! printed in the 

 following year. Bat in the course of thi sixty-three years 

 during which the Association has flourished, it has chanced 

 that a mineralogist has on only one occasion, that of 1S62, 

 been seated in a presidential chair ; and since at that time 

 presidential addresses had no", yet come to be re.;arded as 

 necessary to the existence of the Sections, Prof. Miller refrained 

 from inflicting a mineralogic.il dissertation on an audience 

 which, he had reason to presume, would consist entirely, or 

 almost entirely, of chemists. Perhaps you might be tempted 

 to think that the want of prominence of the mineralogists at 

 our previous meetings has been due to a becoming sense of 

 modesty resulting from the study of that science : this would be 

 a mistake. The fact is that a mineralogical memoir, dealin;.; 

 largely with numerical quantities and involving great v.iriety of 

 experiment and technicality, may be read and studied, but 

 should never be heard ; like the mathematician, the minera- 

 logist despairs of making clear to an audience, especially a 

 mixed one, the bearing of any researches which have been made 

 in his subject. But now that sixty-two years have elapsed since 

 the issue of Prol. Whewell's Report, the time has perhaps .at 

 length arrived when it is advisable, notwithstanding the diflS- 

 culties surrounding an oral treatment of mineralogy, to attempt 

 to give to the Association a faint idea of the present position of 

 the study of the subject. And if most of my hearers find that 

 the remarks are too technical to be in any great part intelligible, 

 let them console themselves with the reflection that, it the 

 future at all resembles the past, only Shalum and Ililpa can 

 have to endure again that particular kin 1 of manvais quart 

 d'liettre which is to precede the geological feast of today. 



Tiie SysUnt! of CryUallisation. — .Vl the time of the publica- 

 tion of Prof. Whewell's report it had already been established 

 by the researches of Rome de I'lsle, Haiiy, Mohs, and Weiss 

 that the position of any single face of any crystal can be exactly 

 defined by means of two sets of quantities : firstly, three lines 

 or axes, of which the lengths and mutual inclinations are 

 characteristic of the substance itself ; secondly, three whole 

 numbers or indices, rarely rising higher in ni.agnitudc than the 

 number 6 : further an empirical arrangement of crystals into 

 systems had been based by Mohs and Weiss on the relative 

 lengths and inclinations of the axes. And a long scries of 

 observations of the optical characters of crystals had revealed 

 to Brewster the fact that the boundaries of the cl.asses of 

 optically isotropic, uniaxal and biaxal crystals form part of the 

 boundaries of the empirical systems. But whereas only three 

 optical classes of crystals had been recognised, it was certain 

 that there were at least four geometrical systems, and it was a 

 matter of controversy as to whether the independence of two 

 others should not be regarded as geometric.illy established. 



The first important discovery following the issue of Whewell's 

 Report was one which proved that the two doubted systems 

 are natural ones. It was found by llerschel and Neumann 

 that the biaxal crystals arc not optically similar, as had hitherto- 

 been supposed, but are of three kinds. In crystals of one kind 

 — for example, barytes — the two lines bisecting the angle of 

 the optic axes internally and externally, and a third line per- 

 pendicular to both, are constant in direction in the crystal 

 whatever the colour of the light ; in a second kind — for instance, 

 selenite — only one of these lines is constant when the colour 

 varies ; in a third kind — for instance, bor.ax — none of the three 

 lines has any constancy of direction. And these three kinds o( 

 biaxal crystal correspond exactly in their f.acial development 

 to the three systems of crystallisation of which ihe independence 

 had already been asserted by some crystallographers on geo- 

 metrical grounds. From this time the arrangement of crystals, 

 into the SIX systems has been regarded as a natural one; and 

 the optical method based on the figures seen in plates when 

 examined in convergent polarised light has been in constant 

 use, and is an invaluable aid in the determination of the system 

 ol crystallisation. 



Ciyilallografhic Notation. — For a simple method of express- 

 ing the relative positions of crystal faces by a symbol, crystallo- 

 graphers are infinitely indebted to the late Prof. Miller, o£ 

 Cambridge. The symbols introduced by Mohs, Weiss, Levy, 

 Naumann, and the modification of the latter suggested by Dana,, 

 though interesting, are not to be compared foi legibility, pro- 

 nounceabilily, or utility in calculation, with the simple symbol- 

 which is associated with the name of Prof. Miller. Tnough 

 Ihe symbol was not invented by him, he was the one who, so- 

 to say, gave it life. He discovered and made known its many 



