Not just the scents are at work here to make your bath more relaxing. Try this project and learn what salts can do. (Adult supervision recommended.)
Note: the salts and glycerin can also be found at drug stores.
Rosemary: stimulating, invigorating, deodorizing
Lavender: deodorizing and antibacterial
Note: Essential oils have different grades and must never be taken internally or applied directly on the skin. Use as directed.
Most likely you found it easier to form lather (and more of it!) when using the water with salt rather than the water with no salt. This is because of the difference between hard water and soft water.
Most households in America have hard water. Hard water has a high mineral content, usually with calcium and magnesium, whereas soft water contains less of these minerals. Calcium and magnesium ions in the hard water react with the soap, forming insoluble gray flakes called soap scum rather than a lather. This means you need more soap to get clean and the bathtub gets a grimy ring around it from the leftover soap scum. One way to soften hard bath water is to add bath salts. The calcium and magnesium ions in the water are replaced with sodium and potassium ions from the salt, allowing the soap to lather much more easily. (If your home has soft water, you may not notice too much of a difference in how well the soap lathers in the water with your bath salts and the water without the bath salts. However, the salt and essential oils will still have a beneficial effect on your skin.)
Another benefit of adding bath salts to your bath has to do with osmosis. Osmosis is the movement of water through a membrane (such as your skin) to achieve equilibrium. Your body contains water and salt, whereas an ordinary bath contains mainly water and very little salt. Therefore, water passes through your skin in an effort to balance the concentration of water and salt in you and in your bath. This excess water causes "pruning" (your fingers and toes wrinkle). Adding bath salts to the water causes a more equal balance of salt and water in both you and in the bath, so less water enters your skin and less wrinkling occurs. Salt is also thought to draw impurities and toxins out of your skin and soothes sore muscles!