THE METHODS OF MICROCHEMICAL QUALITATIVE ANALYSIS 303 
IV. The reagent solution is drawn in a narrow channel across a 
dry film obtained by evaporating to dryness a solution of the sub¬ 
stance to be tested. 
Reactions requiring a nice adjustment of concentration or 
leading to the formation of moderately soluble compounds, thus 
entailing a considerable loss of time waiting for the formation of 
crystals, if much liquid were present, are always best performed 
on the dry residue. Residues for such reactions should consist 
of thin, uniform films of material and are to be obtained only 
when scrupulously clean slides are employed, when only a small 
amount of the substance is present and when care is taken to 
avoid heating too hot during the evaporation. Gentle heating and 
blowing on the warm drop will give the best results. Heating 
should be done at the corner of the object slide over the tiny flame 
of the micro-burner, tipping the object slide so as to cause the 
drop to flow toward the corner and holding above the flame in such 
a position that the tip of the flame is nearer the middle of the 
slide. This prevents the liquid from creeping and from spreading. 
It is usually advisable to examine this film under a low power 
to learn whether it is thin and uniform in character. 
In cases where a ridge of the solid material tends to form around 
the edge, as will be the case if too much substance has been used, 
it is advisable to remove this ridge by means of the platinum 
spatula (Fig. 82), using it shovel-wise. The reagent is dissolved 
in a tiny drop of water placed just beside the dried test drop, and 
is then drawn across the latter with a quick stroke of a glass 
rod with drawn-out end, care being taken to avoid rubbing the 
slide in leading the reagent across. To facilitate the flow, the 
slide should be inclined a trifle in the direction the liquid is being 
drawn. The solution should never spread over the entire film 
of substance, but should remain as a streak of liquid dividing the 
dry spot in half. When the liquid completely covers the residue, 
it is usually due to one or more of several causes: too thick a film; 
a slide that is not clean; heating after the residue was dry and 
so detaching it from the glass; too much reagent, or the presence 
of excessively soluble compounds or those which refuse to adhere 
to the glass. 
