393 FLUORINE. 



and cleaned. If the mineral submitted to the test contained 

 fluorine, the design will be perceived upon the glass ; when the 

 quantity of fluorine, however, is very small, the engraving does 

 not appear immediately, but becomes visible on passing the 

 breath over the glass. The presence of silica in the mineral inter- 

 feres with this operation, but an indication may then be obtained ' 

 by heating a fragment of the mineral to redness upon a piece of 

 platinum foil slipt into a glass tube, 8 or 10 inches in length and 

 open at both ends. The tube is held obliquely with the mineral 

 near the lower end, and so that part of the vapour from the flame 

 passes up the tube. The moisture, thus introduced, carries away 

 the gaseous fluoride of silicon, and condenses in drops in the 

 upper part of the tube. These drops when afterwards evapo- 

 rated, in drying the tube, leave a white spot, which consists of 

 silica, coming from the decomposition of the fluoride of silicon 

 by the water with which it condensed (Berzelius). Dr. G. O. 

 Rees has lately called in question the existence of fluorine in 

 bones, which he finds, contrary to the general opinion, not to 

 be indicated in them by this test. 



Fluoride of boron, fluoboric acid, B F 3 . This compound is 

 gaseous, and is obtained when dry boracic acid is brought in 

 contact with concentrated hydrofluoric acid ; when boracic acid 

 is ignited with fluor spar ; and most conveniently by heating 

 together in a glass retort, 1 part of vitrified boracic acid, 2 of 

 fluor spar and 1 2 of concentrated sulphuric acid, although this 

 process does not give it free from fluosilicic acid. The reaction 

 by which the fluoboric acid is then produced may be thus ex- 

 pressed : 



3 CaF and BO 3 and 3(HO,SO 3 ) = 3(CaO,SO 3 ) and 3HO 

 and BF 3 . 



Fluoboric gas has no action upon glass, and may be collected 

 in glass vessels over mercury. It is colourless, but produces 

 thick fumes when allowed to escape into the atmosphere. Its 

 density according to Dr. J. Davy is 2371, and 2312 according 

 to Dumas, who finds 1 volume of this gas to contain ij vol. of 

 fluorine. Fluoboric gas is not decomposed by iron and the or- 

 dinary metals, even at a bright red heat, but on the contrary, 

 potassium, with the metals of the alkalies and alkaline earths, 

 decomposes it at a red heat ; boron is liberated by potassium, 

 and a double fluoride of boron and potassium also formed. 



