GRAMINEM. 591 



silicate, which seems possible, as they mention traces of 

 crystallization. The remarkable optical properties of this 

 amorphous silicic acid attracted the attention of Brewster 

 {Trmu. Roy. Soc. London (1819), i., 283—299), who found it to 

 possess very little power of refracting light, and to show when 



heated in the dark a brilliant phosphorescence. The informa- 

 tion concerning its physical properties given by Brewster was, 

 however, partly contradicted, and, as it seems, with reason, by 

 Guibourt in 1885. Edward Turner {Ebcnda, pp. 335 — 338) 

 found that the substance examined by Brewster could easily 

 be dissolved in a solution of caustic potash, even after having 

 been heated ; the silicic acid separated from the solution, after 

 being heated to redness, weighed nearly as much as the 

 quantity of Tabashir examined. The transparency which 

 Tabashir acquires when immersed in water was noticed by 

 Brewster and Guibourt; this property is still more striking when 

 it is immersed in a volatile oil or liquid paraffin, for then with 

 Tory pure specimens the outlines are scarcely to be distinguished. 

 Guibourt determined its sp. gr. in water to be 2'149 at 4-°0., 

 and found Indian Tabashir to be composed of 97*39 per cent, of 

 silicic acid, 2'9 per cent, water, with traces of potash and lime. 



In 1859 Fliickiger [ScJuveizerische Zeitsch. /. Pharm., 1859, 

 244) examined a very fine specimen of Tabdshir from Java, 

 where it is known by the name of Bafuginmff, and found it to 

 be almost pure silicic acid. It would appear, however, to be 

 sometimes mixed or adulterated with the ashes of the cane, as 

 I^ost van Tonningen [Jahresh. der Chem., 1860, 531) found a 

 specimen to contain silicic acid 86'38, water 7'63, oxide of 

 iron, potash, and lime 5*99 per cent. 



I^e careful examination of Tabashir made byPoIeck {Zeihch. 

 des dsferr. Apoth. Ver., 3887, p. 139) shows beyond doubt that 

 it may be considered to be silicic acid, although the question 

 remains open, whether it is the normal acid Si(OH*). 

 (Flucliiger, Zur Geschichte des TabascMr, Zeitsch. des allg. 

 <^sfen\ Apoth. Ver. Nr. 14 u. 15, 1887.) As regards the 

 variations observed in the amount of water contained in this 

 substance, the reader is referred to our remarks under Commerce. 



I 



