THE METHODS OF MICROCHEMICAL QUALITATIVE ANALYSIS 311 



Behrens x has successfully used this procedure in testing for 

 the alkaloid quinine. Although no other practical application 

 of this method of testing has yet been made, its possibilities in 

 organic analysis are great, and the principle upon which the test 

 is based is exceedingly interesting, namely, inducing crystalliza- 

 tion in an amorphous mass through the presence of a mother 

 substance dissolved in a suitable solvent. 



XI. Testing for the evolution of gas from a substance when treated 

 with a reagent. 



Dissolve in hot freshly drawn distilled water such an amount 

 of pure gelatin (one or two square millimeters of sheet gelatin) 

 that the solution just jells on cooling. It is essential that this 

 jelly shall not possess too high a setting power nor yet be so thin 

 that considerable time is required for it to set after melting. 



The substance to be tested, if a solution, should be evaporated 

 to dryness in a thin film, or if a solid, very finely powdered or 

 spread out in a thin uniform layer. Upon the dry residue a 

 small drop of the melted gelatin is caused to fall, is quickly 

 spread in a thin layer, and the slide allowed to stand upon a cool 

 metal surface until the gelatin sets. The preparation is then 

 placed upon the stage of the microscope and is focused. Next to 

 the jelly drop is placed the reagent whose effect is to be tested, 

 and by means of the glass rod, the reagent drop is caused to touch 

 the jelly mass. The reagent slowly penetrating into the jelly 

 attacks the substance. If a gas of relatively low solubility is 

 generated tiny gas bubbles will appear in the gelatin. 



Applied as above described the test has a somewhat wider 

 range of usefulness than if the reagent (acid) is dissolved in the 

 gelatin, as suggested by Behrens. 



In the event that the gas set free by the reagent is very soluble 

 in water, no gas bubbles will appear; in such an event the gela- 

 tin may be made the carrier of some reagent upon which the gas 

 will react and be thus made to reveal its presence. 



Usually, however, it is better to drive off the volatile com- 



1 Anleitung, z. mikro. Anal. v. wichtigstcn organ. Verbind. Ilcft III, 92. 



