292 
ELEMENTARY CHEMICAL MICROSCOPY 
and after cooling, gently heat the material in a hot stage of the 
type shown in Fig. 134, until sublimation takes place. The chief 
difficulty with the tube method lies in the fact that the poor 
quality of the glass, the striations, air bubbles, and defects 
render the examination of the sublimate complicated and diffi¬ 
cult. Laying the tube in a drop of oil or of glycerine at the point 
where the sublimate appears facilitates the study, by preventing 
the formation of heavy black contour bands. 
Distillation. Simple as well as fractional distillations are 
as important in the separation and identification of compounds 
in microchemical analysis as in the usual methods on a larger 
scale, and although one of the most difficult of microchemical 
methods may, nevertheless, with care and patience, be performed 
as successfully as the series of fractional distillations on the 
usual scale of the chemical laboratory. 
The simplest of the distillation problems arises in the detec¬ 
tion of a volatile constituent which can be expelled from non¬ 
volatile material by heating after the addition of a suitable 
reagent, as, for example, in the detection of ammonia by expulsion 
from material made alkaline with sodium hydroxide or in the 
detection of inorganic or organic acids set free from their salts by 
phosphoric acid and expelled by heat. The method of procedure 
is as follows: Place in a deep 25-millimeter watch glass a tiny 
bunch of fibrous asbestos wkich has just been ignited to redness 
by being held with the forceps in the flame of a Bunsen burner. 
In the absence of asbestos pure glass wool or in certain cases even 
a piece of filter paper may be employed as the absorbent, but if 
filter paper is employed a blank must always be made to prove 
that no misleading substances result. The asbestos or glass 
wool prevents the spurting and splashing of the liquid. Upon 
the absorbent is placed a small amount of the material to be 
tested, sufficient water and enough expelling reagent to just 
thoroughly moisten the mass but no more. Invert over the 
watch glass thus prepared a glass slide, bearing at its center a 
minute drop of water about i millimeter in diameter which has 
been acidulated or made alkaline as the case requires. Hold the 
watch glass thus covered by grasping its edges between the 
