Although similar in appearance, chemises and slips perform different functions.
Chemises are worn next to the body, typically with a set of drawers. They protected the body from the corset and vice-versa.
Slips are worn over the underwear, as the final layer before the outer garments. They provide a slinky surface for the outer garments to slide over the body, hence the name slip.
While there was some transition, pretty much chemises were worn while corsets were in style, and slips started about the same time corsets started transitioning into girdles.