Liquid media such as broth, milk, litmus milk, indol and nitrate broths are used in the above-mentioned tubes when small quantities only are to be worked with. The tubes are filled approximately one-third full, then plugged with _non-absorbent_ cotton and sterilized. _Cotton plugs_ are used so much in bacteriological work because they permit a free circulation of air and gases and at the same time act as filters to keep out the bacteria of the air. Sugar broths or other media in which gas may be produced are used in fermentation tubes (Smith tubes) of the type shown in Fig. 120 so that the gas may be collected in the closed arm of the tube, measured (Fig. 121) and tested if desired. [Illustration: FIG. 117.--Round wire basket.] [Illustration: FIG. 118.--Square wire basket.] [Illustration: FIG. 119.--Culture tubes with media in them. × 2/3. _1_ to _7_ are the smaller tubes mentioned in the text; _9_ the larger tube; _8_ is extra large for potato tubes; _1_, plain broth; _2_, plain milk; _3_, litmus milk; _4_, gelatin for “stab” or “puncture” culture; _5_, agar for “stab” or “puncture” culture; _6_, agar for “slope” or “slant” culture; _7_, blood serum; _8_, potato tube; _9_, agar for plating. Note the transparency of the broth and gelatin and the slight opalescence of the agar.] One method of using gelatin and also agar is as “puncture” or “stab” cultures. The tubes (the narrower tubes are to be preferred for most “stab” cultures) are filled one-third full of the medium while it is still fluid, plugged, sterilized and allowed to cool in the vertical position. The medium is then “inoculated” with a _straight_ platinum needle by plunging this into the center of the surface down to the bottom of the tube (Fig. 119, Nos. 4 and 5). [Illustration: FIG. 120.--Fermentation tubes. _1_, filled ready for use; _2_, shows a cloudy growth and the development of gas in the closed arm.] Agar and blood serum are frequently used in the form of “slope” or “slant” cultures. That is, the medium solidifies with the tubes lying on their sides which gives a long, sloping _surface_ on which the bacteria are inoculated (Fig. 119, Nos. 6 and 7). [Illustration: FIG. 121.--Method of estimating percentage of gas in a fermentation tube by means of the “gasometer”, the reading is 45 per cent.] [Illustration: FIG. 122.--A toxin flask showing a large surface growth.]