hygroscopic

a. [ Cf. F. hygroscopique. ] 1. Of or pertaining to, or indicated by, the hygroscope; not readily manifest to the senses, but capable of detection by the hygroscope; as, glass is often covered with a film of hygroscopic moisture. [ 1913 Webster ]

2. Having the property of readily inbibing moisture from the atmosphere, or of the becoming coated with a thin film of moisture, as glass, etc. [ 1913 Webster ]