48 APPLIED BACTERIOLOGY 



hours the thermometer will be found to register a few 

 degrees below that at which you desire to work. To bring 

 the heat up to a proper working temperature you will now 

 require to move the lead weight L on the lever M (which 

 until now has been quite to the left) step by step towards 

 the right, an inch or so at a time, and to observe the effect 

 on the thermometer at hourly or half-hourly intervals. 

 When the thermometer indicates the required temperature, 

 clamp the weight to the lever. 



The rate of flow of water is not very important. We 

 have found a flow of 120 drops a minute sufficient to 

 correct an external difference of 10°, but a fluid ounce per 

 minute may be run through without ill effect, because as 

 soon as the temperature for which the apparatus is ad- 

 justed is arrived at, the water falls between the two tubes and 

 simply runs to waste. 



The screw valve K on the top of the apparatus is for ad- 

 justing the water-supply, and the cock below the boiler ia 

 for emptying the apparatus. 



The valve K should be permanently connected with a 

 constant supply of water from a tank above the level of the 

 apparatus. The usual house-supply will generally be found 

 to answer the purpose perfectly, but it must be clearly 

 understood that if the water ceases to flow there will be no 

 regulation of temperature. The waste-pipe N should also 

 be carried to a properly-trapped drain. 



In climates where the external air is always above the 

 temperature of the incubating chamber, the ice-box must be 

 so frequently replenished that there is always ice in it, 

 without which there is of course no cooling effect. 



A lamp or glass may be used for heating the boiler, but 

 need not be lighted unless the air is below the temperature 

 required in the incubating chamber. 



The Centrifugal Machine.— This machine has many useful 



