264 ELEMENTARY CHEMICAL MICROSCOPY 



and by means of a 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. 



EXPERIMENTS. 



a. Evaporate a drop of Na2COa solution. Cover with gelatin, test with HC1. 



b. Place a little CaCOs on an object slide, cover and test as above. 



c. Test a little zinc dust in like manner. 



d. Test a cyanate in like manner, using H 2 SO 4 . 



XII. An amorphous precipitate is formed by the reagent and 

 requires special treatment to induce crystallization. 



It has already been pointed out that in microchemical quali- 

 tative analysis an amorphous precipitate is the least desirable 

 form in which a substance may be separated for identification. 

 Nevertheless, it often happens that such precipitates are obtained 

 either accidentally or when it is more expedient to thus remove 

 a substance in order to prevent it from interfering in subsequent 

 testing for other substances. 



In qualitative analysis by means of microscopic methods two 

 classes of amorphous precipitates are met with : (a) Those which 

 require solution in a special solvent from which a crystalline 

 compound eventually separates, and (b) those in which crystal- 

 lization can be induced by inoculation with a trace of the same 

 compound in a crystalline condition. 



Special mention is here made of the treatment of amorphous 

 precipitates because in a number of instances treatment with hot 

 concentrated sulphuric or hydrochloric acids must be resorted 

 to in order to obtain recognizable compounds. 



