Neptune Bath Salts were a personal care product available to consumers on the planet Magna Roma in the mid-23rd century, a chemical preparation of mineral salts designed to be dissolved in bathwater to soothe the condition and aroma of the Magna Roman bather's skin.
In the year 2268, Captain James T. Kirk read a copy of The Gallian magazine which contained an advertisement for Neptune bath salts, taking note of the propensity for the Magna Romans to name things after the culture of the ancient Roman Empire. Named for the god Neptune, Neptune bath salts were advertised alongside Mars toothpaste and the Jupiter 8 automobile. Septimus, a Magna Roman rebel leader, noted that the products mentioned in the media were named for "false gods", from his point of view as a Son worshipper. (TOS episode & Star Trek 8 novelization: Bread and Circuses)