STERILISED POTATOES. 



57 



by soaking them in a half per cent, solution of perchloride of 

 mercury for three quarters of an hour ; they are then rinsed in 

 distilled water and are submitted to the 

 action of the "steam steriliser," as 

 devised by Koch (Fig. 14). This con- 

 sists of a cylindrical tin receiver, pro- 

 vided with a conical lid, and covered 

 completely with a closely fitting jacket 

 of felt, to avoid too rapid radiation of 

 heat. Internally, at the junction of the 

 lower and middle thirds of the re- 

 ceiver, is a perforated diaphragm, which 

 separates the lower or water chamber 

 from the upper or steam chamber. The 

 water chamber is furnished with a strong 

 floor to withstand the heat of a powerful 

 gas burner applied beneath it ; and the 

 steam thus generated in the lower 

 chamber passes through the perforated 

 diaphragm, and fills the steam chamber 

 above. The water chamber is provided 

 with a side register tube to indicate the 

 depth of water present in it, which should 

 never be less than three or four inches. 

 The conical lid of the steriliser is per- 

 forated at its apex to admit the stem 

 and bulb of a thermometer, which regis- 

 ters the temperature to which the con- 

 tents of the steam chamber are subjected. 



Into the upper chamber can be lowered the potato-holder, a 

 cylindrical tin receptacle, with a perforated floor, which fits loosely 

 within the steam steriliser, and, resting on the perforated diaphragm, 

 allows its contents to be exposed to the steam as it rises from the 

 lower to the upper chamber. The potato-holder is charged with the 

 requisite number of previously purified potatoes, and is placed in the 

 steriliser when the contained steam has reached close upon 100° C. 

 The potatoes are left for thirty minutes (a few minutes more or less 



Fig. 14.— "Steam steriliser." 



a. Diaphragm. 



b. Water chamber. 



c. Steam chamber, in which 



is potato receiver with 

 perforated floor. 



