STERILIZATION BY DISCONTINUOUS HEATING 13 



for which it is required. Hence it is better to have some form of water 

 bath, in which the temperature is automatically controlled by a regulator 

 on the gas supply. 



[1. Hearson's water bath. This is a very convenient and reliable form 

 of water bath. It consists of a cylindrical copper vessel (fig. 8) heated 

 below by an ordinary fish-tail gas burner, the temperature being controlled 



FIG. 8. Hearson's water bath. 



by a capsule attached to the outside of the bath, and through which the 

 gas passes. The capsule has a range of about 10 C., and within these limits 

 the temperature is regulated by means of a milled screw. 



[Technique. Pour sufficient water into the bath to reach above the level 

 of the regulator outside. Put on the lid, and pass a thermometer through 

 the hole in it, being careful to see that the bulb is in the water. Light the 

 gas. The temperature gradually rises until it reaches the point for which 

 the regulator is set : the gas is then automatically lowered and the tem- 

 perature remains stationary. To raise the temperature, turn the screw 

 clockwise, to lower it, contra-clockwise. 



[Note. It must be remembered that the capsule has a working limit of 

 about 10 only, the exact limits being indicated when the instrument is 

 supplied. Consequently, if a bath is required to work sometimes at 

 55-65 C., and at other times at 75 85 C., it is necessary either to have 

 two baths, or a single bath in which capsules are interchangeable. 



[Be careful always to see that the water is above the level of the top of 

 the capsule, and when filling the bath never add water of a temperature 

 higher than that for which the capsule is regulated.] 



2. Weissnegg's water bath. Another form of bath, which is shown in fig. 9, consists 

 of a metal vessel fitted with a Roux's regulator placed in a side chamber, the heat 

 being supplied by a gas burner below. 



