From Virginibus Puerisque by Stevenson:
Hope is the boy, a blind, headlong, pleasant fellow, good to chase swallows with the salt; Faith is the grave, experienced, yet smiling man.
I did some googling, but nothing meaningful seems to come up. I have two vague guesses about either birds being chased and caught somehow using salt, very strange, or some alcoholic drinks being supplemented with salt, but how might that relate to Hope is beyond my grasp.
P.S. Could it mean to "chase things in life that have flavour (salt) in them" ? To chase delights, in other words.