. Manual of bacteriology. Bacteriology. BACTERIAL FERMENTATIONS IN SUGARS. 79 V:f't. i Fig. used is a gas-forming one, then, as growth occurs, little bubbles appear round the colonies. These frequently coalesce to form bubbles of a larger size, and those which are super- ficial in process of time diffuse out of the medium. This method is very fre- quently used for studying gas formation by B. coli. (2) Durham's Tubes (Fig. 44, ^). — The plug of a tube which contains about one-third more than usual of a liquid medium is re- moved, and a small test- tube is slipped into the latter mouth dow
. Manual of bacteriology. Bacteriology. BACTERIAL FERMENTATIONS IN SUGARS. 79 V:f't. i Fig. used is a gas-forming one, then, as growth occurs, little bubbles appear round the colonies. These frequently coalesce to form bubbles of a larger size, and those which are super- ficial in process of time diffuse out of the medium. This method is very fre- quently used for studying gas formation by B. coli. (2) Durham's Tubes (Fig. 44, ^). — The plug of a tube which contains about one-third more than usual of a liquid medium is re- moved, and a small test- tube is slipped into the latter mouth downwards. The plug is replaced and the tube sterilised thrice at 100° C. The air re- maining in the smaller tube is thereby expelled. The tube is then inoculated with the bac- terium to be tested. Any gas developed collects in the upper part of the inner tube. (3) The Fermentation Tube (Fig. 44, c).—This consists of a tube of the form shown, and the figure also indicates the extent to which it ought to be filled. It is inoculated in the bend with the gas-forming organism, and when growth occurs the gas collects in the upper part of the closed limit, the medium being displaced into the bulb. H. W. Hill has advantageously modified the ordinary fer- mentation tube of Smith (Fig. 45) by having the bulb made larger so as to accommodate twice the quantity of fluid contained in the branch, thus avoiding wetting of the plugs during sterilisation; also, in having replaced the sealed end of the branch by a snugly fitting, hollow, ground-glass thimble, or stopper, permitting one to examine the contents of the branch, either chemically or bac- teriologically, without contamination by fluid in the bulb, such as happens when using the ordinary form of the tube. In carrying ' out this examination it is obviously necessary to first replace Tubes for demonstrating gas-formation by bacteria, ff, tube with " shake" culture. b, Durham's fermentation tube. c, ordinary form of fermentat
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