426 ELEMENTARY CHEMICAL MICROSCOPY 
sheaves of acicular prisms appear and iater there are formed 
long thin prisms with square ends, giving polarization colors 
and parallel extinction. Nitron nitrate has a very low solubility 
even in warm water, hence the reaction is a delicate one. The 
sheaves of white crystals, appearing brownish by reflected light, 
are characteristic. 
In dilute solutions none of the salts of the common acids inter¬ 
fere save iodides and bichromates. With these salts there may 
be obtained crystals which closely resemble the nitrate but these 
crystals disappear upon even gentle warming; nitron nitrate will 
not. 
From concentrated solutions there may be obtained under 
favorable conditions, precipitates with chlorates, perchlorates, 
phosphates, chromates, bichromates, iodides, ferro- and ferri- 
cyanides, oxalates and tartrates, but in no case in dilute solu¬ 
tions with gentle warming should there be any difficulty in 
differentiating between such precipitates and the crystals ob¬ 
tained with nitrates. 
NITRITES. 
a. With Silver Nitrate there is obtained a felted mass of fine 
needles with long acicular prisms at the outer edge of the mass, 
changing into short stout prisms with imperfectly developed 
ends. These crystals are colorless under the microscope and do 
not show their greenish tint until viewed in masses by reflected 
light. 
h. With Potassium Iodide and Starch. Add to the drop to be 
tested a crystal of potassium iodide, then a little potato starch 
and finally a trace of dilute sulphuric acid. The hydroiodic 
acid set free by the acid is oxidized by the nitrous acid; iodine 
is liberated and stains the starch blue or violet or black. 
Always test the potassium iodide, with starch and dilute sul¬ 
phuric acid, to ascertain its purity and to be certain that no 
appreciable blueing of the starch takes place with the reagents 
alone. 
Only traces of iodine are liberated from iodide when treated 
