Abstract: A fail safe control circuit in which an input signal for initiating control of a potentially dangerous system is amplified only if it lies between given limits and also only if electrical power is being supplied to the system. The outputs of the amplifiers are monitored to check their correct operation, and two independent optical output signals obtained therefrom. The optical signals are arranged independently to operate switches in the system, such that both switches have to operate before the system can be actuated.

Abstract: Apparatus for the chemical synthesis of an evaporant and a liquid has a cryogenic pumping device between the evaporant source and the reaction region and a pump causes the liquid to be continuously recirculated until the reaction is completed, the pressure in the evaporation region being maintained substantially lower than that in the reaction region by differential pumping.

Abstract: Chemical synthesis apparatus includes a differential pumping device mounted in an evacuable container between an evaporant source providing a first chemical constituent and the reaction region. The differential pumping can be provided by apertured diaphragms or hollow cones, which may be cooled, mounted directly above the source which is thereby maintained at a lower pressure than the reaction region. The source may be an electron beam source or a resistively heated source arranged to direct the evaporant upwards into the reaction region and a second chemical constituent may be introduced into the reaction region. The invention also provides a method of chemical synthesis in which differential pumping is carried out between the reaction region and the source of one of the constituents.