▁heat , ▁and ▁is ▁known ▁as ▁the ▁ent hal py ▁of ▁fusion ▁and ▁is ▁exactly ▁the ▁same ▁as ▁the ▁energy ▁required ▁to ▁m elt ▁the ▁same ▁amount ▁of ▁the ▁solid . ▁ ▁Low - temper ature ▁hel ium ▁is ▁the ▁only ▁known ▁exception ▁to ▁the ▁general ▁rule . ▁Hel ium - 3 ▁has ▁a ▁negative ▁ent hal py ▁of ▁fusion ▁at ▁temper atures ▁below ▁ 0 . 3 ▁K . ▁Hel ium - 4 ▁also ▁has ▁a ▁very ▁slightly ▁negative ▁ent hal py ▁of ▁fusion ▁below ▁ 0 . 8 ▁K . ▁This ▁means ▁that , ▁at ▁appropriate ▁constant ▁press ures , ▁heat ▁must ▁be ▁added ▁to ▁these ▁subst ances ▁in ▁order ▁to ▁free ze ▁them . ▁ ▁Vit r ification ▁ ▁C ertain ▁materials , ▁such ▁as ▁glass ▁and ▁g ly cer ol , ▁may ▁hard en ▁without ▁cry st all izing ; ▁these ▁are ▁called ▁am orph ous ▁sol ids . ▁Am orph ous ▁materials , ▁as ▁well ▁as ▁some ▁polym ers , ▁do ▁not ▁have ▁a ▁free zing ▁point , ▁as ▁there ▁is ▁no ▁ab rupt ▁phase ▁change ▁at ▁any ▁specific ▁temperature . ▁Instead , ▁there ▁is ▁a ▁grad ual ▁change ▁in ▁their ▁vis co el astic ▁properties ▁over ▁a ▁range ▁of ▁temper atures . ▁Such ▁materials ▁are ▁character ized ▁by ▁a ▁glass ▁transition ▁that ▁occurs ▁at ▁a ▁glass ▁transition ▁temperature , ▁which ▁may ▁be ▁roughly ▁defined ▁as ▁the ▁" k ne e " ▁point ▁of ▁the ▁material ' s ▁density ▁vs . ▁temperature ▁graph . ▁Because ▁vit r ification