This potion is actually a light oil which is applied externally to the dress and exposed flesh. 

It then confers etherealness. 

In the ethereal state,  the individual can pass through solid objects - sideways, upwards, downwards - or to different planes. 

Naturally, the individual cannot touch non-ethereal objects. 

The oil takes effect 3 rounds after application and it lasts for 4 + 1-4 turns unless removed with a weak acidic solution prior to the expiration of its normal effective duration. 

It can be applied to objects as well as creatures; one potion is sufficient to anoint a normal human and such gear as he or she typically carries (two or three weapons, garments, armor, shield, and the usual miscellaneous gear carried). 

Ethereal individuals are invisible. 

(Cf. phase door spell, and The Known Planes of Existence.)