148 ELEMENTS OF APPLIED MICROSCOPY. 
are diverse and striking. In some cases the mixture of 
the two materials remains liquid longer han 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 
Fic. 56.—Contact ZONE BETWEEN CHLORIDES OF SILVER AND IoDIN. 
(After Lehmann.) 
either of the pure substances may form in the «ontact 
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, 
