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. TrRIpHYLITE [(Fe, Mn, Li*)® P? O%]. ' 
At Grafton in our state this rare species is found more abundantly, 
*A. Streng, Tschermak’s Mineralogische Mittheilungen, 1876: Heft iii, p. 166. 
