Water-bath. For the purpose of observing the thermal death-point a special water-bath is necessary. The temperature of this piece of apparatus is controlled by means of a capsule regulator that can be adjusted for intervals of half a degree centigrade through a range of 30°, from 50° C. to 80° C. by means of a spring, actuated by the handle a, which increases the pressure in the interior of the capsule. A hole is provided for the reception of the nozzle of a blast pump, so that a current of air may be blown through the water while the bath is in use, and thus ensure a uniform temperature of its contents. Through a second hole is suspended a certified centigrade thermometer, the bulb of which although completely immersed in the water is raised at least 2 cm. above the floor of the bath. Sterile glass capsules. Flask containing 250 c.c. sterile normal saline solution. Case of sterile pipettes, 10 c.c. (in tenths of a cubic centimetre). Special platinum loop. Test-tubes, 18 by 1.5 cm., of thin German glass. Case of sterile petri dishes. Tubes of agar or gelatine. METHOD.-- 1. Prepare tube cultivations on solid media of optimum reaction; incubate forty-eight hours under optimum conditions as to temperature and atmosphere. 2. Examine preparations from the cultivation microscopically to determine the absence of spores. 3. Pipette 5 c.c. salt solution into each of twelve capsules. 4. Suspend three loopfuls of the surface growth (using a special platinum loop, _vide_ page 316) in the normal saline solution by emulcifying evenly against the moist walls of each capsule. 5. Transfer emulsion from each capsule to sterile 250 c.c. flask, and mix. 6. Pipette 5 c.c. emulsion into each of twelve sterile test-tubes numbered consecutively. 7. Adjust the first tube in the water-bath, regulated at 40° C, by means of two rubber rings around the tube, one above and the other below the perforated top of the bath, so that the upper level of the fluid in the tube is about 4 cm. below the surface of the water in the bath, and the bottom of the tube is a similar distance above the bottom of the bath. 8. Arrange a control test-tube containing 5 c.c. sterile saline solution under similar conditions. Plug the tube with cotton-wool and pass a thermometer through the plug so that its bulb is immersed in the water. 9. Close the unoccupied perforations in the lid of the water-bath by means of glass balls. 10. Watch the thermometer in the test-tube until it records a temperature of 40° C. Note the time. Ten minutes later remove the tube containing the suspension, and cool rapidly by immersing its lower end in a stream of running water. 11. Pour three gelatine (or agar) plates containing respectively 0.2, 0.3, and 0.5 c.c. of the suspension, and incubate.