ished ▁and ▁et ched ▁samples ▁under ▁the ▁optical ▁mic ros cope . ▁Although ▁such ▁methods ▁enabled ▁the ▁collection ▁of ▁a ▁great ▁deal ▁of ▁empir ical ▁evidence , ▁particularly ▁with ▁regard ▁to ▁factors ▁such ▁as ▁temperature ▁or ▁composition , ▁the ▁lack ▁of ▁cry stal log raph ic ▁information ▁limited ▁the ▁development ▁of ▁an ▁understanding ▁of ▁the ▁fundamental ▁physics . ▁ ▁Nevertheless , ▁the ▁following ▁became ▁well - est ab lished ▁features ▁of ▁gra in ▁growth : ▁ ▁Gra in ▁growth ▁occurs ▁by ▁the ▁movement ▁of ▁gra in ▁boundaries ▁and ▁not ▁by ▁co ales c ence ▁( i . e . ▁like ▁water ▁dro ple ts ) ▁ ▁B ound ary ▁movement ▁is ▁dis continu ous ▁and ▁the ▁direction ▁of ▁motion ▁may ▁change ▁suddenly . ▁ ▁One ▁gra in ▁may ▁grow ▁into ▁another ▁gra in ▁whilst ▁being ▁consum ed ▁from ▁the ▁other ▁side ▁ ▁The ▁rate ▁of ▁consumption ▁often ▁increases ▁when ▁the ▁gra in ▁is ▁nearly ▁consum ed ▁ ▁A ▁cur ved ▁boundary ▁typically ▁migr ates ▁towards ▁its ▁centre ▁of ▁curv ature ▁ ▁When ▁gra in ▁boundaries ▁in ▁a ▁single ▁phase ▁meet ▁at ▁angles ▁other ▁than ▁ 1 2 0 ▁degrees , ▁the ▁gra in ▁included ▁by ▁the ▁more ▁ac ute ▁angle ▁will ▁be ▁consum ed ▁so ▁that ▁the ▁angles ▁approach ▁ 1 2 0 ▁degrees . ▁ ▁D riv ing ▁force ▁The ▁boundary ▁between ▁one ▁gra in ▁and ▁its ▁neighbour ▁( gra in ▁boundary ) ▁is ▁a ▁defect ▁in ▁the ▁cry stal ▁structure ▁and ▁so ▁it ▁is ▁associated ▁with ▁a ▁certain ▁amount ▁of ▁energy