64 
MICRO-CHEMICAL EXAMINATION OF MINERALS. Mar., 1889 . 
use) produces the precipitation of the double sulphate of 
cerium and sodium, in the form of a cloud of minute granular 
masses, without any distinct crystalline form. Potash 
produces a similar double sulphate, but it separates some¬ 
what later and in much larger granules. Behrens describes 
them as somewhat similar in appearance to the starch grains 
of potato. The reaction is obviously the usual one adopted 
for the separation of the cerium metals, and as the other 
bases which usually accompany ceria form similar compounds 
with the sulphates of soda and potash, though not so readily, 
the purity of the cerous sulphate is not imperative—an 
important fact, as the salt which is to be obtained of the 
dealers in chemicals mostly contains a quantity of the allied 
bases, the purification from which is a difficult matter. 
For Magnesia we test by adding a drop of dilute hydro¬ 
chloric acid, then dilute ammonia, and then connecting with the 
drop by a narrow channel another drop of water in which has 
been placed a small lump of microcosmic salt. The distance 
of the drops should be about -1- of an inch, the reason 
being that the slow mixing of the solutions is essential to the 
formation of the characteristic crystals, different at the two 
ends, of the phosphate of magnesia and ammonia. In the 
presence of iron and alumina we must wait after the addition 
of ammonia till these bases are precipitated, and they then do 
not interfere. 
Keciprocallv, an ammoniacal solution of magnesia may be 
used to test for Phosphoric Acid. As, however, we have 
frequently to test for this acid when occurring in apatite 
crystals, which are so enveloped in other minerals that the 
products of decomposition and solution are mixed together, 
recourse is frequently had to a solution of molybdate of 
ammonia in nitric acid. The mineral should be attacked with 
nitric acid, and the solution mixed on a glass slip with the 
molybdic solution. The presence of phosphoric acid is shown 
by yellow octohedra and rhombic dodecahedra of the ammonic 
phosphomolybdate. 
Our tests of Lithia are not very satisfactory, but carbonate 
of potash is said to give fair results. On the other hand very 
small proportions of lithia are easily distinguished by means of 
the spectroscope — even one of the small direct vision 
spectroscopes which are made for the pocket. A fragment of 
the mineral heated with gypsum in the zone of fusion of the 
Bunsen burner shows at once the intense red line, lying 
between the principal lines, due to soda and potash. The 
carmine colour, which pure lithia salts impart to the flame, 
is completely masked by a very minute quantity of soda, but 
