METHOD OF SEDGWICK AND TUCKER 109 



7. Sedgrwick and Tucker's method — 



Sedgwick employs as a filtering medium finely granulated 

 sugar in the special form of tube illustrated. This consists of a 

 glass tube of about 30 cm. in length, half of which has a bore of 

 2-5 cm. and the other half a bore of -5 cm. A plug of glass 

 wool is inserted at a to act as a support to the filtering material, 

 and the tube is then sterilised at 160' C. for one hour in the hot 

 air steriliser. When cool the narrow portion of the tube is filled, 

 as far down as the glass wool plug and up to its opening into 

 the lumen of the larger tube, with finely granulated cane sugar. 

 Cotton-wool plugs are fitted into the orifices of both tubes and 

 the whole is then again sterilised for two hours at 130'' C. If a 

 higher temperature than this is reached there will be danger 

 of melting the sugar, and forming caramel. Great care should 

 be taken that the sugar is in a thoroughly dry condition before 

 it is inserted into the tube. When the apparatus is required 

 for use, the cotton-wool plug of the smaller tube is removed 



Fig. 33. — Sedgwick's tube. 



and the tube itself connected up with the aspirator. The second 

 cotton-wool plug is withdrawn and a measured quantity of 

 air drawn through the tube. The wool plugs are then replaced. 

 On arrival at the laboratory the sugar is shaken down to the wider 

 portion of the tube, or pushed down by means of the central plug of 

 glass wool, and 1 5 c.c. of melted nutrient gelatine are poured into 

 the larger end, after which the two wool plugs are replaced. The 

 sugar readily dissolves in the melted gelatine, and the tube is then 

 rolled on ice or under a cold water tap in order to fix the gelatine 

 in an even surface round its inner walls. The colonies will appear 

 in two to three days according to the temperature of incubation. 

 Unless very numerous their subsequent enumeration is compara- 

 tively easy, but some difficulty is experienced when it is required 

 to pick out the different colonies for examination. Another objec- 

 tion to this method is that the rapid liquefaction of the gelatine 

 under the influence of liquefying bacteria will, at times, render 

 the counting and the isolation of particular organisms a matter of 

 extreme difficulty. 



