Dr Fleming an the Neptunian Formation of Stalactites. 307 



4. A micrometer of several concentric rings, which may be 

 illuminated in the dark field. This micrometer has four eye- 

 glasses. 



5. An achromatic finder, of thirty inches in focal length, and 

 twenty-nine of aperture. 



6. An instrument for correcting the axis of the great object- 

 glass. 



The price of the telescope now described is about 8000 

 Prussian dollars, or nearly L. 1300 Sterling. The total weight 

 of the whole package was thirty-eight quintals. 



An achromatic telescope, with an object-glass eighteen feet 

 in focal length, and with an aperture of twelve inches, and fur- 

 nished with eye-glasses, micrometers, and parallactic stand, 

 like the one now described, amounts to about L. 2720 Ster- 

 ling. 



Mr Fraunhofer engages likewise to construct these instru- 

 ments with object-glass eighteen inches in diameter ; and, as 

 the price increases nearly as the cube of the diameter, an in- 

 strument of this kind will cost about L. 9200 Sterling. 



Art. XXIV. — On the Neptunian Formation of Siliceous 

 Stalactites* By the Rev. John Fleming, D. D. F- R. S. E. 

 Communicated by the Author. 



1 he formation of siliceous minerals has perplexed, in no or- 

 dinary degree, the different sects of Geologists. The Vul- 

 canists and Volcanists have, in vain, attempted to repel the 

 objection brought against their views, founded on the refrac- 

 tory nature of silica in the fire, amounting to infusibility. 

 The Neptunians have been equally embarrassed with its cha- 

 racter of insolubility in aqueous menstrua. Fluxes have 

 been resorted to by the former, and solvents by the latter, 

 without any thing satisfactory to the unprejudiced having 

 been announced. It is not to be concealed, however, that 

 several facts in the natural history of siliceous minerals, which 

 have been established, lead to the conclusion that solvents of 

 silica do exist in nature, though these solvents be yet un- 



* Read before the Royal Society of Edinburgh, March 7, 1825. 



