Also other silicone-copolymer membranes other than the one described in the preferred embodiment may be used, for example, silicone rubber membranes such as the homopolymeric dialkylsiloxanes and copolymers of dialkylsiloxane and siloxanes of the type RR'SiO where R is a monocyclic aryl radical and R is alkyl or monocyclic aryl.