A lithographic apparatus includes an illumination system to illuminate a pattern of a patterning device and a projection system to project an image of the pattern onto a substrate. The illumination system includes a first and second enclosures, a scaling device, and a protective device. The first enclosure encloses a first environment and includes a first opening and first connection corresponding to the first opening. The second enclosure includes a second connection structure to couple to the first connection structure to prevent mixing of substances between the first environment and a second environment outside of the first and second enclosures. The sealing device is disposed between the first and second connection structures. The material of the sealing device is chemically reactive to the first environment. The protective device is disposed on the sealing device proximal to the first environment to shield the sealing device from the first environment.