148 



ELEMENTS OF APPLIED MICROSCOPY. 



are diverse and striking. In some cases the mixture of 

 the two materials remains Hquid longer fhan the pure 

 substances, on each side, forming a clear area in which, 

 later, an amorphous precipitate is deposited. The chlo- 

 rides of iodin and silver exhibit this case to perfection 

 (Fig. 56). Or a crystalline precipitate different from 



a. b. L 



Fig. 56. — Contact Zone between Chlorides of Silver and Iodin. 



(After Lehmann.) 



either of the pure substances may form in the -contact 

 zone, as with silver and potassium iodides. Or the 

 crystals may grow across the line unchanged, forming 

 a single homogeneous mass. This occurs with identical 

 bodies and with isomorphous substances like dibrom- 

 benzol and dichlorbenzol. Or the crystals may grow 

 across the contact zone, but with spaces between them, 

 due to the presence of impurities in one or other of the 

 substances. Usually the habitus of the crystals in the 

 impure substance will be somewhat changed. Pure 

 dinitrotoluol and the same body containing paranitro- 

 toluol exhibit this phenomenon. These four cases by no 

 means exhaust the possible conditions of the contact zone, 

 but they serve as types of some of the more striking ones, 



