328 
ELEMENTARY CHEMICAL MICROSCOPY 
Since the crystals usually lie on one of the faces of the octa¬ 
hedron, there is apt to result an abnormal development of this 
face and the diagonally opposite and parallel face; the resulting 
crystal will thus exhibit an hexagonal outline when seen through 
the microscope, i.e., viewed from above. Combinations of cube 
and octahedron may lead to a somewhat similar appearance. 
Not infrequently preparations are obtained in which twinning 
is very marked, and others in which there is a grouping of crys¬ 
tals in threes or fours. Of the twin crystals, one form seems to 
predominate; it results from the union, in reversed position, of 
two halves of an octahedron where the dividing plane is parallel 
to the two opposite faces. 
The size and rate of development of the crystals formed will 
depend largely upon the concentration of the test drop. In 
very concentrated solutions, minute crystalline grains or the 
skeletons of octahedra are produced. In very dilute solutions 
the crystals appear only after some time. In case the test drop 
proves to be of the latter sort, heat it gently to cause slight 
evaporation, or expose to alcohol vapor, see Method VI, page 
305* 
Thin crystals are lemon yellow in color, but those which attain 
a considerable thickness are of a decided orange tint. 
The best results are obtained from neutral solutions or those 
which are very slightly acid with hydrochloric acid. Excess of 
mineral acids is to be avoided, sulphuric acid in particular. 
Either evaporate and remove them, or mitigate their action by 
adding sodium acetate or sodium carbonate. If the latter salt 
is used, care should be taken to avoid making an alkaline solu¬ 
tion and a large excess of the chloroplatinic acid must always 
be used. 
Ammonium, rubidium, cesium and thallous-thallium also give 
octahedral crystals with chloroplatinic acid, the composition of 
the salts being similar to that of the potassium salt. The 
solubility of these compounds, and consequently the size of 
the crystals produced, decreases rapidly in the order in which 
the elements are named. Ammonium will give octahedra of the 
same size as those of potassium, hence its absence must be 
