490 



NA TURE 



\_March 24, 1887 



of lime and vegetable matter. He failed to find any trace 

 of alkalies in it. 



In 185s, Giiiboiirt {Joiini. Jc Pharin. [3], xxvii. 81, 161, 

 252 ; Phil. J/<!f. [4], .N. 229) analysed a specimen of 

 tabasheer having a specific gravity of 2T48. It gave the 

 following result : — 



Silica = 96 '94 



Potash and lime = o'i3 

 Water = 2 93 



Organic matter = trace 



Guibourt criticised some of the conclusions arrived at 

 by Brewster, and sought to explain the source of the silica 

 by studying the composition of different parts of the 

 bamboo. While the ashes of the wood contained 00612 

 of the whole weight of the wood, the pith was found to 

 contain 044S per cent., the inner wood much less, and 

 the greatest proportion occurred in the external wood. 

 On these determinations Guibourt founded a theory of 

 the mode of formation of tabasheer based on the sugges- 

 tion that at certain periods of its growth the bamboo 

 needed less silica than at other times, and that when not 

 needed, the silica was carried inwards and deposited in 

 tlie interior. 



In the year 1S57, D. W. Rost van Tonningen, of Buiten- 

 zorg, undertook an investigation of the tabasheer of Java, 

 which is known to the natives of that island under the 

 name of" singkara" {Naticrku?idig Tijdsclirift voor Necier- 

 liindsch Indie, vol. xiii., 1S57, p. 391)- The specimens 

 examined were obtained from the Bantbusa apus growing 

 in the Residency of Bantam ; it is described as resembling 

 in appearance the Indian tabasheers. Its analysis gave 

 the following result : — 



Silica = 86-387 



Iron oxide = 0-424 



Lime = 0*244 



Potash = 4 806 



Organic matter = 0-507 

 Water — 7-632 



Total ... looooo 



Apart from the question of its singular mode of origin, 

 however, and its remarkable and anomalous physical 

 properties, tabasheer is of much interest to mineralogists 

 and geologists. All the varieties hitherto examined, with 

 the exception of the peculiar one from the Andes, are 

 in composition and physical characters true opals ; this is 

 the case with all the Indian and Java varieties. They 

 consist essentially of silica in its colloidal form, the w-ater, 

 lime, potash, and organic matter being as small and vari- 

 able in amount as in the mineral opals ; and, as in them, 

 these substances must be regarded merely as mechanical 

 impurities. 



The tabasheers must be studied in their relations on 

 the one hand with certain varieties of tlie natural semi- 

 opals, hydrophanes, beekites, and floatstones, some of 

 which they closely resemble in their physical characters, 

 and on the other hand with specimens of artificially de- 

 posited colloid silica formed under different conditions. 

 Prof. Church, who has so successfully studied the 

 beekites, informs me that some of those remarkable 

 bodies present singular points of analogy with tabasheer. 



By the study of thin sections I have, during several 

 years, been endeavouring to trace the minute structure of 

 some of these substances. In no class of materials is it 

 more necessary to guard one's self against errors of ob- 

 servation arising from changes induced in the substance 

 during the operations which are necessary to the pre- 

 paration of transparent sections of hard substances. 

 Unfortunately, too, it is the custom of the natives to 

 prepare the substance for the market by an imperfect 

 calcination, and hitherto I have only been able to study 

 specimens procured in the markets which have been 

 subjected to this process. It is obviously desirable, before 



attempting to interpret the structures exhibited under the 

 microscope, to compare the fresh and uncalcined materials 

 with those that have been more or less altered by heat. 



Tabasheer would seem, from Brewster's experiments, to 

 be a very intimate admixture of two and a half parts of 

 air with one part of colloidal silica. The interspaces 

 filled with air appear, at all events in most cases, to be so 

 minute that they cannot be detected by the highest powers 

 of the microscope which I have been able to employ. It 

 is this intimate admixture of a solid with a gas which 

 probably gives rise to the curious and anomalous properties 

 exhibited by this singular substance. 



The ultra-microscopical vesicles filled with air in all 

 probability give rise to the opalescence which is so marked 

 a property of the substance. Their size is such as to 

 scatter and throw back the rays at the blue end of the 

 spectrum and to transmit those at the red end. 



When the vesicles of the substance are filled with 

 Canada balsam, and a thin slice is cut from it, this 

 opalescence comes out in the most striking manner ; 

 very thni sections are of a rich orange-yellow by trans- 

 mitted light, and a delicate blue tint by reflected light. 



Section of Indi.in tabasheer, seen with a magnifying power of 

 250 diameters. 



1 tlo not know of any substance w-hich in such thin films 

 displays such striking opalescence. 



That the excessively low refractive po-.ver of tabasheer 

 is connected with the mechanical admixture of the col- 

 loidal silica with air seems to be proved by the experi- 

 ments of Brewster, showing that with increase of 

 density there was an increase m the refractive index, 

 from riii in specimens of the lowest specific gravity, to 

 1-182 in those of the highest specific grivity. Where 

 the surface was hard and dense, Brewster found the 

 refractive index to approach that of semi-opil. The 

 wonderful thing is that a substance so full of cavities 

 containing gas should nevertheless be transparent. 



By the kindness of Mr. F. Rutley, F.G.S., I am able 

 to supply a drawing taken from one of my sections of 

 tabasheer. 



The accompanying woodcut gives some idea of the 

 interesting structures exhibited in some sections of taba- 

 sheer, though much of the delicacy and fidelity of the 

 original drawing has been lost in transferring it to the 

 wood. 



In this particular case, the faint punctation of the 

 surface may possibly indicate the presence of air-vesicles 



