560 



Proceedings of the Royal Irish Academy. 



in the mineral. ^N'o doubt can be entertained of the title of evansite 

 to specific rank, although its claim has not been duly recognized in 

 mineralogical text-books.^- I give here the mean percentages obtained 

 by Forbes from his three analyses, and the corresponding numbers de- 

 manded by the formula he adopted : — 





Experiment. 



Theory, 

 3Al„03,P,05,i8H20. 



Alumina, 



Phosphorus Pentoside, . 



Water, 



Silica, 



Loss, 



39-31 



19-05 



39-95 



1-41 



•28 



39-78 

 18-35 



41-8" 



100-00 100-00 



1 



On placing the finely-powdered mineral over sulphuric acid in vacuo 

 it lost little more than traces of moisture : but at 100°, or rather in 

 the Tvater-oven, a very different result ensued. Here are the figures : — 



Anal. 11. "37 gram evansite lost in vacuo in 36 hours 



•004 ,, II2O. At 100° it sustained a further loss of 

 •0795 ,, HoO. On moderate ignition the remainder, 

 •0685 ,, H2O, was evolved. 



The percentages of water to which the above results correspond are as 



foUow : — 



Loosely combined water, lost at 100° 22-56% 

 'Water, lost on ignition, .... 18" 51% 



Total water, 41-07% 



Now if evansite contains I8H2O, the loss of 



10 HoO corresponds to 23-26% 



8 HoO 



18-61% 



41-87% 



These numbers leave no doubt as to the peculiar position occupied by 

 evansite amongst the aluminium phosphates. In accordance with the 



^2 Eammelsherg's Mineral-Chemie, 2iid ed., vol. ii., p. 320. 



