The apparatus comprises a distributor device having two inlets connectible to two sources for supplying the gases to be mixed and a plurality of outlets each of which is connected to a first end of a respective capillary tube. The distributor device is arranged to connect each capillary tube to one or other of the two supply sources so that mixtures of the two gases are formed in a mixing chamber into which the other ends of the capillary tubes open, the mixtures containing the two gases in proportions which differ according to a number of capillary tubes traversed by each gas. The apparatus includes control means for causing the sequential formation of mixtures containing different proportions of the two gases and the supply of these mixtures to an outlet from the apparatus.
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