38 AIDS TO BACTERIOLOGY 



bubbles are seen at the surface, or can be seen rising in the 

 liquid if the tube is gently shaken. As gas production 

 may have stopped at the time of inspection, it is never 

 wise to dispense with the inner tube. 



Maltose, galactose, arabinose, raffinose, cane-sugar, 

 mannite, sorbite, dulcite, adonit, dextrin, starch, and 

 inulin, are similarly used. 



Milk. Fresh separated milk, free from preservatives, 

 is sterilised for twenty minutes on each of five successive 

 days. The medium is frequently tinged with litmus. 



Milk must not be overheated, as this retards or prevents 

 the formation of clot when organisms that should produce 

 clotting are grown in it. This, perhaps, is the reason why 

 some brands of non- sweetened condensed milk prove 

 unsatisfactory for milk-tubes. Milk is always so greatly 

 contaminated to start with that it is unwise to rely on 

 sterilisation, and tubes should be incubated at blood-heat 

 for two days in order to ascertain which are sterile. 



Potato-Tubes. Large sound potatoes are thoroughly 

 scrubbed, and then with a large cork-borer cylindrical 

 pieces are cut to fit into test-tubes. Each cylinder is cut 

 into halves diagonally, the wedges are well washed in 

 running water for an hour, and each wedge is placed in a 

 test-tube. The cores of potato rest on a moist plug of 

 cotton- wool to keep the potato cylinder moist. The tubes 

 are capped, and sterilised in the steam steriliser for thirty 

 minutes on each of three successive days. The tubes are 

 left in the blood-heat incubator overnight, and any con- 

 taminated ones rejected. 



Blood-Serum. Blood from the jugular vein or an 

 incised wound is allowed to run into a tall sterile glass 

 vessel, with aseptic precautions. The vessel is at once 

 placed in a cool place without the least shaking, and 

 allowed to stand overnight, when a firm clot forms; the 

 clear serum is drawn off by a sterile glass siphon or large 

 pipette into sterile test-tubes, which are plugged, and laid 

 on a slanting surface, and the serum made to set by 

 heating in the hot-air steriliser to 65 C. The tubes can 

 then be sterilised in the usual way by steaming on three 

 successive occasions. All tubes should be tested for 

 sterility by a trial incubation before use. The serum 

 should have a jelly-like consistency, and an opalescent, 

 yellowish-white colour. 



