Penfield and Foote — Composition of Tourmaline, 109 



The ratios of Si0 2 to the total hydrogen atoms show a wide 

 variation, and the hydrogens for the most part fall far below 

 20, which is required by the new formula. Rammelsberg, 

 however, claims that boron plays the part of a metal, replacing 

 aluminium, and that all tourmalines are derivatives of an acid 

 H 6 Si0 5 or H 24 Si 4 O 20 . Replacing, therefore, the boron by hydro- 

 gens (B 2 8 is equivalent to 3H 2 0), it is found that the ratios 

 of Si0 2 to the total hydrogens then vary from 4 : 21*9 to 4 : 25*1. 

 The result of grouping the ratios is as follows : 



Limits of variation, 21*9 — 22-5 22*5 — 23*5 23'5 — 24-5 24*5 — 25-1 

 Number of analyses, 3 10 17 4 



The ratio of Si0 2 to the total hydrogens demanded by 

 Rammelsberg' s formula is 4 : 24, and in only seventeen, or one- 

 half, of the analyses are the ratios 24 ± 0*5, while they are 

 reasonably sharp, 24 ± 0*2, in only seven cases. It would seem 

 therefore that owing to certain defects the analyses fail to give 

 decisive results. The ratios do not satisfactorily support 

 Rammelsberg's formula nor the one which we have proposed. 

 In his latest publications* Rammelsberg claims great accuracy 

 both for his analyses and for the analytical methods which he 

 employed, and in justice to him it does not seem right to 

 ignore his results without endeavoring to discover the possible 

 defects of the analyses and the influences exerted by them 

 upon the ratios. In the first place, it is believed, for the fol- 

 lowing reasons, that his silica determinations are too high : it is 

 presumable that facilities were not at hand for producing the 

 intense heat necessary for expelling the last traces of water 

 from the silica ; there is no statement that the silica was tested 

 as to its purity by volatilization with hydrofluoric acid ; and, 

 lastly, a little silica might have been derived from glass beakers 

 and even from the distilled water, provided it was kept in glass 

 bottles or carboys. It is certain that the water determinations 

 are too low, and this is admitted by Rammelsberg. It is well 

 known that when a silicate containing fluorine and hydroxyl is 

 heated some hydrofluoric acid is liberated, and if the experi- 

 ment is tried in a closed tube the glass is etched, while proba- 

 bly the mineral also is attacked with the formation of silicon 

 fluoride. Rammelsberg,f however, points out that only a por- 

 tion of the fluorine is thus expelled, and since the amount of 

 fluorine in tourmaline is small, the loss on ignition should give 

 a fair estimate of the amount of water which the mineral con- 

 tains. Loss on ignition seems to have been very carefully 

 determined by Rammelsberg, and in those analyses in which 



* Berlin Akademie, 1890. Mineralchemie, Zweites Supplement, 1895. 



f Mineral Chemie, Zweites Supplement, p. 284. 



