British Mineralogy. 345 



2" 8-1) was first broken up in small pieces; and after well rubbing 

 the pieces in a cloth and throwing- away the dust in order to be 

 secure against the introduction of any trace of metallic iron from 

 the employment of an iron hammer, these were finely pulverized 

 in a porcelain mortar. By means of a magnet, all attractable 

 particles were then removed from this powder, and, after a careful 

 washing in order to remove as much adherent earthy matter as 

 possible, the metallic particles were then carefully dried, using 

 alcohol, so as not to permit of their becoming oxidated, and re- 

 served for examination. 



The researches of Andrews having shown that the postcreta- 

 ceous basaltic rocks of Antrim contain iron in the metallic 

 state, a fact since confirmed by Pagels in the case of the basalt 

 of Barenstein in Germany, a portion of the mineral was treated 

 with a solution of acid sulphate of copper, precisely as described 

 by Professor Andrews, but no indication of the presence of me- 

 tallic iron was discovered. 



Although the metallic particles now obtained appeared to the 

 naked eye as if quite free from any admixture, it was not found 

 possible (notwithstanding several further attempts by repulve- 

 rizing, extracting by the magnet, and again washing) to obtain 

 the mineral perfectly free from adherent rock -matter, since under 

 the microscope it appeared to the last as a black opake mineral 

 to which small particles of a colourless transparent silicate were 

 attached. 



In order, therefore, to arrive at as near an approximation to 

 its true specific gravity as possible, the following method was 

 adopted. 



The specific gravity was first determined on a weighed amount 

 of the mineral, which was then acted upon by boiling sulphuric 

 acid until all the oxide of iron and titanic acid was extracted, 

 leaving the soluble siliceous matter behind, which was weighed, 

 and subsequently its specific gravity was determined as usual. 

 These data were found to be as follows : — 24*82 grs. original mi- 

 neral of specific gravity3'65 afforded 8*84grs. of rock admixture*, 

 of specific gravity 2*601, and consequently, by the calculation 



2*601 x 3-65 x 15*98 151*5085270 _ 

 (2*601 x 24*82) - (3*65 x 8*84) 32*29082 



the required specific gravity of the mineral. 



The chemical examination was conducted as follows : — A 

 weighed amount, previously reduced to impalpable powder, was 

 repeatedly digested in fresh portions of sulphuric acid in a pla- 



* This amount includes 1*17 gr. alumina, lime, and magnesia, found to 

 be present in the course of the analysis. 



Phil Mag. S. 4. Vol. 34. No. 231. Nov. 1867. 2 A 



