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 which 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 



