"Evanesce" means "To fade out of sight, 'melt into thin air,' disappear." The word is traced back to 1855, to Frederick William Faber's "Growth in Holiness":
We must with holy superstition keep [spiritual favors] secret.... As soon as they are known they will evanesce. This is their way. When God means us to let any of them be known, He will give us such a light that we cannot mistake Him and such an impulse that we cannot resist Him.This feels weighty, given the inclusion, earlier today, of "knowing God" as the 3rd item on my list of "List of Easy and Impossible Enterprises."