Apparatus for mixing of a first gas and a second gas, including (i) a linear first tube for supply of the first gas, the first tube having an inlet for the first gas at an upstream end and tapering to form a nozzle at a downstream end, and (ii) a linear second tube for supply of the second gas. The second tube has an inlet for the second gas at an upstream end, a first portion which forms an annulus around the outer surface of the first tube upstream of the nozzle, and a second portion which forms a sheath around the nozzle of the first tube and which forms an area of expanded cross-section compared to the annulus.