2. The oxygen may be displaced by some other gas, and though coal-gas, nitrogen, and carbon dioxide may all be used for this purpose, it has become the almost universal practice to grow anaërobes in _hydrogen_. The production of the hydrogen is readily obtained by Kipp's or some other suitable apparatus for the generation of hydrogen from zinc and sulphuric acid. The free gas is passed through various wash-bottles to purify it of any contaminations. Lead acetate (1-10 per cent. water) removes any traces of sulphuretted hydrogen, silver nitrate (1-10) doing the same for arseniated hydrogen; whilst a flask of pyrogallic acid will remove any oxygen. It is not always necessary to have these three purifiers if the zinc used in the Kipp's apparatus is pure. Occasionally a fourth flask is added of distilled water, and this or a dry cotton wool pledget in the exit tube will ensure germ-free gas. From the further end of the exit tube of the Kipp's apparatus an india-rubber tube will carry the hydrogen to its desired destination. With some it is the custom to place anaërobic cultures in test-tubes, and the test-tubes in a large flask having a two-way tube for entrance and exit of the hydrogen; others prefer to pass the hydrogen immediately into a large test-tube containing the culture (Fränkel's method). Either method ends practically the same, and the growth of the culture in hydrogen is readily observed. Yet another plan is to use a yeast flask, and after having passed the hydrogen through for about half an hour, the lateral exit tube is dipped into a small flask containing mercury. The entrance tube is now sealed, and the whole apparatus placed in the incubator. The interior containing the culture is filled with an atmosphere of hydrogen. No oxygen can obtain entrance through the sealed entrance tube, or through the exit tube immersed in mercury. Yet through this latter channel any gases produced by the culture could escape if able to produce sufficient pressure. [Illustration: KIPP'S APPARATUS For the Production of Hydrogen] [Illustration: FRÄNKEL'S TUBE For Cultivation of Anaërobes]