▁of ▁great ▁er ud ition , ▁combining ▁a ▁knowledge ▁of ▁phil ology ▁with ▁history ▁and ▁eth n ology . ▁His ▁German ic ▁studies ▁had ▁taught ▁him ▁the ▁necessity ▁of ▁knowing ▁C elt ic ▁languages , ▁and ▁he ▁went ▁to ▁work ▁to ▁investigate ▁this ▁neglect ed ▁field . ▁To ▁get ▁at ▁the ▁sources , ▁the ▁old ▁manuscript s , ▁particularly ▁those ▁in ▁Old ▁Irish , ▁he ▁journey ed ▁to ▁Kar ls ru he , ▁Wür zburg , ▁St . ▁G allen , ▁Milan , ▁London , ▁and ▁Oxford , ▁and ▁everywhere ▁made ▁extract s ▁or ▁copies . ▁Both ▁ancient ▁and ▁modern ▁dialect s ▁received ▁his ▁attention . ▁His ▁Gram mat ica ▁C elt ica ▁proved ▁beyond ▁doubt ▁that ▁the ▁C elt ic ▁languages ▁were ▁a ▁group ▁of ▁the ▁Ind o - Europe an ▁family , ▁which ▁put ▁C elt ic ▁phil ology ▁on ▁a ▁sound ▁scientific ▁basis . ▁After ▁the ▁author ' s ▁death ▁the ▁work ▁was ▁re vised ▁and ▁re - ed ited ▁by ▁Hermann ▁E bel ▁( Ber lin , ▁ 1 8 7 1 ). ▁Other ▁works ▁by ▁Ze uss ▁are ▁Trad ition es ▁possession es que ▁Wir zen burg enses ▁( 1 8 4 2 ) ▁and ▁Die ▁Fre ie ▁Reich stadt ▁Spe yer ▁vor ▁ihrer ▁Z erst ör ung ▁( The ▁Free ▁Imperial ▁City ▁of ▁Spe yer ▁before ▁its ▁Dest ruction ). ▁ ▁De ▁Bar ra ▁( 2 0 1 8 , ▁page ▁ 2 6 ) ▁writes ▁that ▁Ze uss ▁" br ought ▁rig orous ▁scientific ▁method ologies