396 



NA TURE 



\_Fch. 24, i< 



sciences of Nature must be content to recognise.individuals 

 as the only real entities, and to accept species, like genera, 

 families, and orders, as convenient but purely artificial 

 conceptions. 



The geological study of minerals leads us to regard 

 each specimen that we examine as possessing a dis- 

 tinguishing combination of properties, some of which are 

 impressed upon it by causes operating when it came into 

 being, while others are no less clearly the result of 

 the long series of vicissitudes through which it has since 

 passed. 



Of all the branches of mineral morphology there is none 

 from the study of which the geologist has gained more 

 in the past, or from which he has greater reason to look 

 for future aid, than that of the embryology of crystals. 



In the year 1S40 Link showed that the first step in 

 the formation |of crystals in a solution consists in the 

 separation of minute spherules of supersaturated liquid in 

 the mass ; and subsequently Harting in Holland, and 

 Rainey and Ord in this country, obtained a number of 

 interesting experimental results, by allowing crystallisa- 

 tion to take place slowly in mixtures of crystalloids and 

 colloids. 



Valuable contributions to the same subject were made 

 by Frankenheim, Leydolt, and others ; but it is to Her- 

 mann Vogelsang that we owe the greatest and most im- 

 portant contributions to mineral-embryology. By the 

 mgenious device of adding viscous substances to solutions 

 in which crystallisation was going on, he succeeded in so 

 far retarding the rate at which the operation took place as 

 to be able to study its several stages. He thus showed 

 how the minute " globulites," gathering themselves into 

 nebulous masses or ranging themselves according to 

 mathematical laws, gradually build up skeleton-crystals, 

 by the clothing of which the perfect structures arise. 



Since the early and regretted death of Vogelsang, the 

 subject of the development of crystals from their embryos, 

 the so-called ctystallilcs, has been successfully prosecuted 

 by Behrens, Otto Lehmann, Wichman, and other in- 

 vestigators. 



Now in all glasses — whether of natural or artificial origin 

 — in which the process of primary devitrification is going 

 on, we have examples of the growth of crystals in a viscous 

 and retarding mass, and in these, as Leydolt, Zirke), 

 and Vogelsang clearly saw, admirable opportunities are 

 afforded to us for studying the formation of crystallites, 

 and the laws which govern the union and growth of these 

 into crystals. Two years ago, my predecessor in this 

 chair submitted to you the interesting results of his own 

 researches upon the devitrification of artificial glasses 

 and slags ; and the subject has since been pursued by 

 Velain in France, and by Hermann and Rutley in this 

 country. 



The igneous rocks supply us with admirable oppor- 

 tunities for studying mineral embryology. In the same 

 rock-mass we may sometimes find every possible gradu- 

 ation, from an almost perfect glass to a holocrystalline 

 aggregate. By the study with the microscope of the 

 several transitions in different parts of the mass, we 

 obtain data for the most important conclusions concerning 

 the phenomena of crystal-development. 



There is another line of research in connection with 

 mineral-embryology, which appears to be full of promise, 

 and which has not yet received all the attention it deserves 

 In the " contact-zones '• around great igneous intrusions, 

 we find the curious so-called " spotted slates," which under 

 the microscope are seen to contain nebulous patches, the 

 mere ghostly presentments of crystals, struggling into 

 being in the amorphous mass. The development of these 

 nebulous masses into perfect crystals, exhibiting the 

 characteristic external forms and optical properties of 

 andalusite and kyauite, of garnet and epidote, of horn- 

 blende and mica, may be tra ed in some cases with the 

 greatest facility. 



More complicated still are the phenomena exhibited 

 along the foliation-planes of the rocks, which have been 

 made to flow in the act of mountain-making. There, as 

 the old minerals are destroyed, new ones build themselves 

 up from their elements. The study of all the steps of this 

 process is an undoubtedly difficult one, but the results 

 already obtained by Reusch, Lossen, Heim,and Lehmann, 

 by Lapworth, Teall, Roland Irving, and Williams, lead us 

 to look hopefully forward to the full solution of the grand 

 but complicated problems of regional metamorphism. 



The field of mineral-embryology is indeed a promising 

 one, and its diligent cultivators may hope to gather a 

 harvest no less rich than that which has been reaped by 

 the workers in the same department of the biological 

 sciences. 



( To be continued^ 



TABASHEER 



I HAVE often wondered that this curious substance has 

 never attracted more attention. But scanty references 

 to it are to be found in books, and yet it seems to me that 

 few more singular things are to be met with in the 

 vegetable kingdom. 



In Watts's " Dictionary of Chemistry " (vol. v. p. 653), 

 exactly six lines are devoted to it. It is defined to be : 

 " Hydrated silica, occurring in stony concretions in the 

 joints of the bamboo. It resembles hydrophane, and when 

 thrown upon water does not sink till completely saturated 

 therewith.'' It is further stated to be the least refractive of 

 all known solids, and an analysis by Rost von Tonningen 

 of a specimen from Java gives a composition of 86'39 per 

 cent, silica soluble in potash, 4'Si potash, 7'63 water, with 

 traces of ferric oxide (to which I suppose its occasional 

 yellowish colour to be due), lime, and organic matter. 



There are several specimens in the Kevv Museums, 

 partly derived from the India Museum. All consist of 

 small irregular angular fragments, varying from the size of 

 a pea downwards, and opaque white in colour. It is 

 obvious that these fragments are the debris of large 

 masses. 



Now, the presence of considerable solid masses of so 

 inert a substance as hydrated silica in the plant-body is a 

 striking fact. At first sight, one might compare it do the 

 masses of calcium phosphate which form the endo-skeleton 

 in the higher animals. These, however, serve an obvious 

 mechanical purpose, which cannot be attributed to the 

 lumps of tabasheer in the hollow joints of a bamboo. 

 The presence of silica may sometimes serve an adaptive 

 purpose, as in the beautiful enamelled surface of canes. 

 And according to Dr. Vines (•' Physiology of Plants," p.21), 

 " .Struve found that it constitutes 99 per cent, of the dry 

 epidermis of Calamus Rolaiig'' ' 



In a few other groups of plants, such ^iEguiseluin and 

 the Diatoiiiaci-cT, it is a characteristic constituent. In all 

 cases it principally occurs in the cell-wall (Vines, I.e. p. 

 137. This has suggested the highly ingenious speculation 

 that, seeing the mtimate chemical relationship which 

 ob'ains between silicon and carbon, there might be a 

 silicon-cellulose. I notice that Count Castracane, 

 in his Report on the Diatamaceie collected by the 

 Ckalh'iigcr, speaks of its "having been already shown 

 that silica is sometimes substituted for carbon in the 

 formation of cellulose " (p. 7). Judging from ash-analyses 

 it might be supposed that silica was an essential constituent 

 of gramineous plants. But by the method of water- 

 culture Sachs has found that maize, for example, will 

 grow with only a trace of silica. 1 must confess to 

 Ignorance of all that may have been done m the matter 

 recently. But Ladenburg thought, and I think with 

 i-eason, that the indifference of the plant to silica was a 



' Sachs rem.irks[" Text-book, "second evJition,p. 703) that silica accumulates 

 chiefly in the tissues exposed to evaporation, th jugh this clearly does not 

 apply to the case of diatoms. 



