A Three-Dimensional Model of Dendritic Structure There is a charncleristie cryslal growth patleru thnl bears the label "dendritie" r n "pine tree." This pine tree design is freqnenlly seen in metal solidification, i n some geological formations, and in some polymer crystals. A two-dimensiond drawing of a symmetrical pine tree uuunlly serves to describe the s t n ~ c t w e . I n an exploratory st,~rdyof cryntal growth in gels; il, was noted that ammonium chloride iu xgnr gel p~.oduccsa crystal pat,tern that could well serve aq a model of the dendrit.ic structure. I t is a model that can be handled, has nltrnorous variations, and cat, he readily produced from common laboratory materials. The eel was areusred bv heatine 110 e of ammminm chloride in 200 ml of wat,er t,o 90°C
After standing a f m days the wystal patien) h e e m w fnsay due 1.0 flllctuation of room temperature. The pattern can be reproduced by dissolving the crystals while the gel remainr firm. This can be done by immewiug vials i t ) wat,er a t X " C for 20 min and allowing to stand while a lnew cryslal grows. If vialo are kept stoppered, this procedure can be repeated many times.
496 / Journal o f Chemical Educafion
