3. _The Absorption Method._ Instead of adding hydrogen to the tube or flask containing the anaërobic culture, it is feasible to add to the medium some substances, like glucose or pyrogallic acid, which will absorb the oxygen which is present, and thus enable the anaërobic requirement to be fulfilled. To various media--gelatine, agar, or broth (the latter used for obtaining the toxins of anaërobes)--2 per cent. of glucose may be added. Pyrogallic acid, or pyrogallic acid one part and 20 per cent. caustic potash one part, is also readily used for absorptive purposes. A large glass tube of 25 cc. height, named a Buchner's cylinder, having a constriction near the bottom, is taken; and about two drachms of the pyrogallic solution are placed in the bottom of it. A test-tube containing the culture is now lodged in the upper part above the constriction. The apparatus is now placed in the incubator at the desired temperature, and the contained culture grows under anaërobic conditions. As the pyrogallic solution absorbs the oxygen it assumes a darker tint. [Illustration: BUCHNER'S TUBE For Cultivation of Anaërobes] 4. _Mechanical Methods._ These include various ingenious tricks for preventing an admittance of oxygen to the culture. An old-fashioned one was to plate out the culture and protect it from the air by covering it with a plate of mica. A more serviceable mode is to inoculate, say, a tube of agar with the anaërobic organism, and then pour over the culture a small quantity of melted agar, which will readily set, and so protect the culture itself from the air. Oil may be used instead of melted agar. Another mechanical method is to make a deep inoculation and then melt the top of the medium over a bunsen burner, and thus close the entrance puncture and seal it from the air. 5. _Absorption of Oxygen by an Aërobic Culture._ This method takes advantage of the power of absorption of certain aërobic bacteria, which are planted over the culture of the anaërobic species. It is not practically satisfactory, though occasionally good results have been obtained. 6. _Lastly, there is the Air-pump Method._ By this means it is obviously intended to extract air from the culture and seal of it _in vacuo_. The culture tubes are connected with the air-pump, and exhausted as much as possible. Of these various methods it is on the whole best to choose either the hydrogen method, the vacuum, or the plan of absorption by grape-sugar or pyrogallic. In anaërobic plate cultures grape-sugar agar plus 0.5 per cent. of formate of soda may be used. The poured inoculated plate should be placed over pyrogallic solution under a sealed bell-glass and incubated at 37° C. Pasteur, Roux, Joubert, Chamberland, Esmarch, Kitasato, and others have introduced special apparatus to facilitate anaërobic cultivation, but the principles adopted are those which have been mentioned. THE QUALITATIVE ESTIMATION OF BACTERIA IN THE SOIL