124 MINERALOGY AND LITHOLOGY. 



are explained by the supposition that during the cooling of the rock, 

 when it had reached the temperature at which some minerals could crys- 

 tallize, from some reason an elevation of temperature took place, and 

 the crystals were again partially dissolved. To such a cause the odd 

 outlines of these apatite crystals may be referred. 



From such microscopic proportions, apatite increases in size till it can 

 sometimes be seen in the rock with the unaided eye. For example : a 

 porphyritic diorite at Dixville notch is very black in color ; but through 

 it run fine white needles, which, with the lens, appear clear and glassy, 

 and which it requires no microscope to recognize, nor to see how they 

 pierce through all the other minerals which are porphyritically developed. 

 As phosphoric acid is one of the essential constituents of plant food, the 

 wide and universal distribution of apatite may be regarded as fortunate. 



If it is wished to make certain that a little hexagonal crystal in a rock 

 section is apatite, one may use the reaction which Streng* applied to 

 distinguish apatite from nepheline. 



Upon a crystal in a section with an uncovered surface, place a drop of 

 a concentrated nitric acid solution of molybdate of ammonia, and watch 

 the reaction with the microscope. The nitric acid will gradually decom- 

 pose the apatite crystal, and in the drop there will presently appear a 

 precipitate of the ammonium-phospho-molybdate, which has a bright yel- 

 low color, and is composed of little crystals which are either octahedral 

 or dodecahedral. This reaction cannot fail to be recognized, since this 

 precipitate contains only 3.6 per cent, of phosphoric acid, and is corre- 

 spondingly bulky ; moreover, it is soluble in an excess of phosphoric 

 acid, and hence directly over the exposed crystal no precipitate will be 

 seen, but the precipitate will surround this spot with a crystalline wreath. 

 Again : if a crystal be treated in like manner with a little drop of nitric 

 acid, and, after it is well decomposed, a tiny bit of sulphuric acid be added, 

 a precipitate of sulphate of lime will form ; or, if the crystal be treated 

 directly with sulphuric acid, its exposed surface will be quickly covered 

 with a white coat of the same, which will prevent all further action. 



83. Triphylite [(Fe, Mn, Li^)^ P^ 0«]. 

 At Grafton in our state this rare species is found more abundantly, 



* A. Streng, Tschtrmak' s Mineraiogische Mittheilungen, 1876: Heft iii,p. 166. 



