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 film 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- 



