Abstract: A gas mixer is disclosed which is used for mixing different gases in order to obtain a solid by sublimation or chemical reaction. The mixing is carried out in a mixing chamber having a uniform, substantially circular, cross section and a length depending on the particular application. Build up of the solid is prevented by injecting the gases to be mixed into the mixing chamber so that the combined flow in the duct, at its inception, is characterized by an axial flow of one gas surrounded by a spiralling flow of another gas. Thus, the gases to be mixed will first meet in the vicinity of the center of the duct at what may be termed a shear layer and the shear layer will progressively spread outwardly to the mixer surfaces downstream of the point of injection of the gases into the mixing chamber. The bulk of the solid formation in the duct is in the vicinity of the center of the duct which prevents the formation of solid on or near the mixer surfaces where it would tend to adhere or deposit and cause build ups.