278 
ELEMENTARY CHEMICAL MICROSCOPY 
ration, then remove it from the flame and blow gently upon the 
warm drop, heat again and again blow; repeat the process until 
the solvent has been driven off. If this method is followed, a 
uniform, closely adhering fihn will result instead of irregularly 
distributed loose particles, and the danger of loss through de¬ 
crepitation of the tiny solid particles is avoided. 
It is essential to remember that it is impossible to obtain slips 
made of sufficiently resistant glass upon which water will not 
exert a marked solvent action; moreover, it must constantly be 
borne in mind that all liquids soon take up foreign matter from 
the bottles in which they are kept. The results of tests for 
solubility should always be checked by comparison with the 
residues left when the solvent alone is evaporated under exactly 
the same conditions. 
It follows therefore that tests for the solubility of substances 
in boiling liquids or in strong acids, alkalies, etc., should be per¬ 
formed on clean, bright platinum foil; the solvent is decanted, 
concentrated and only transferred to a glass or quartz slip when 
evaporated almost to dryness. 
Should the illuminating gas be of very poor quality and the 
heating prolonged, an amount of various ammoniacal, sulphur 
and other products may be absorbed by the solvent sufficient to 
vitiate the results. 
If the substance whose solubility is being tested is subse¬ 
quently to be analyzed, a sufficient quantity of it is tested on 
glass, quartz or platinum, according to the necessities of the case, 
care being taken to observe the precautions given above as to 
impurities in solvents and the probability of their action on the 
microscopic slides used. This action may not always be due to 
the solvents alone, but may be the result of the material being 
tested. When more than one solvent has been found, the choice 
will, of course, be governed by many circumstances. It is 
obvious that no fixed rule may be given which will apply to even 
a majority of cases. Much must always be left to the judgment 
of the analyst. 
Decantation. — For most purposes, it is generally possible to 
obtain sufficiently clear solutions from drops containing precipi- 
