32 GEOLOGICAL MEMOIRS. 



and that, by realizing in experiment the conditions suggested by geo- 

 logical observations, artificial apatite is easily obtained. 



By passing a current of the vapour of perchloruret of phosphorus 

 over caustic lime in a porcelain tube submitted to a dull red heat, 

 after a reaction accompanied by a most vivid incandescence, there is 

 formed chloruret of calcium and tribasic phosphate of lime. A por- 

 tion of the chloruret of calcium remains free ; another portion com- 

 bines with the phosphate, and gives a chloro-phosphate, insoluble in 

 water and in acetic acid, which has exactly the composition of na- 

 tural apatite. 



Under the microscope this chloro-phosphate appears as little 

 hexagonal prisms : it has then not only the composition, but also 

 the crystalline form of natural apatite : its density is 2*98, which is 

 a little less than that of natural apatite, and this arises without doubt 

 from the latter containing the fluoruret, instead of the chloruret of 

 calcium, in a predominating quantity ; the former of which has a 

 density much greater than the latter. 



If slacked lime or common carbonate of lime (chalk) be used in- 

 stead of caustic lime, apatite is still obtained. 



Magnesia treated in the same manner as the lime gives an anhy- 

 drous phosphate of magnesia, crystallized in a form derived from the 

 right rhomboidal prism. But this phosphate does not retain the 

 chloruret in the state of combination. This difference between the 

 behaviour of magnesia and lime may explain how it is that magnesian 

 apatite has not been found where the ordinary apatite so frequently 

 occurs. Wagnerite, or magnesian apatite, indeed, has not been met 

 with hitherto but at one locality in the Salzburg. 



Alumina and the aluminate of sodium, treated like the lime, do not 

 yield a compound answering to apatite. 



Silex heated to redness in presence of a current of chloruret of 

 phosphorus is very easily decomposed, and furnishes chloruret of si- 

 licium, which is disengaged with the chloruret of phosphorus. The 

 facility of the decomposition of silex by chloruret of phosphorus fur- 

 nishes apparently a convenient method for preparing the chloruret 

 ofsilicium. 



Having described the formation of a substance closely resembling 

 topaz, obtained by submitting alumina at a white heat to the action of 

 a current of fluoruret of silicium, and having noticed other bodies 

 more or less resembling certain micas, chondrodite, and scheelite as the 

 results of other combinations, M. Daubree goes on to say — these two 

 of the minerals characteristic of tin-ores, apatite and topaz, originate, 

 like the crystallized oxide of tin itself, in the decomposition of chlo- 

 rurets and fluorurets, and by a procedure differing from those hitherto 

 employed in imitating these minerals. All these reactions confirm 

 the theory of metalliferous veins which I proposed in 1841, viz. that 

 they are due to the decomposition of volatile chlorurets and fluorurets 

 arising from profound depths. 



The author then alludes to the probably important part played by 

 these gases in the metamorphosis of certain rocks, such as the topaz- 

 rocks of Saxony, of Villa Rica in Brazil, where the topaz is pene- 



