In modern Japanese, the kana ゑ/ヱ (we) and ゐ/ヰ (wi) are largely obsolete. Words formerly containing them now are pronounced with /e/ and /i/, so they were replaced by え and い, respectively. But when ...
The particle は is pronounced similarly to わ (unlike the rest of the ハ行 kana) when used as a particle; why is this? What historical shifts went on to cause this irregularity?