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LABORATORY EXERCISES IN BACTERIOLOGY. 



more confinement than afforded by the ordinarily fitted lid. If the objects subjected 

 to the steam-bath are not liable to injury from long exposure, the sterilization may be 

 accomplished by a single steaming for an hour ; but ordinarily, to prevent injury and 

 for greater certainty of result, the interrupted method of Tyndall is followed, the arti- 

 cles, after subjection to heat for fifteen or twenty minutes, being removed from the bath, 

 and the process repeated twice at intervals of twenty-four hours. 



(b) Another form of steam sterilizer often employed is the Arnold sterilizer (Fig. 6), 

 which possesses the advantage of quick generation of steam, an advantage often much 

 appreciated in laboratory work. As shown in the diagram in section, this apparatus 

 consists of a steam chamber placed over a thin plate boiler, and communicating 

 with the latter by an open cylinder through which the steam generated in the boiler 



is conducted into the chamber. A water 

 reservoir surrounds this cylinder and com- 

 municates with the boiler beneath by means of 

 several small feed tubes for the supply of 

 water as it is vaporized. In the bottom of 

 the steam chamber a perforated false bottom 

 is placed for the accommodation of the articles 

 to be subjected to sterilization. A light cover 

 is applied to the steam chamber, loosely 

 enough to allow the moderate escape of ex- 

 cess of steam. A hood of metal covers the 

 chamber, in the interior of which the escaping 

 steam is caught and condensed, running down 

 its sides and dropping back into the reservoir, 

 to be used over again. In use, the reservoir 

 should have water poured into it until it 

 stands about half or three-quarters of an inch 

 in depth over the bottom, the boiler being 

 thus also filled. A moderate-sized burner is 

 placed near the middle of the bottom, and in 

 a few minutes a clicking sound is heard, indi- 

 cating the boiling of the water. The articles 

 to be sterilized are now placed in the interior, 

 the cover and hood adjusted, and after a few 

 minutes more a sterilizing heat has been at- 

 tained. Exposure in this apparatus should 

 be the same as in the case of any other form 



of apparatus. After proper exposure, if one is sterilizing such substances as fabric, or 

 tubes plugged with cotton stoppers, it is well in this, as in other forms of steam steril- 

 izers, to give free vent to the steam by raising the cover somewhat, lest the contained 

 steam, in cooling, condense upon the material and render it wet. A caution should be 

 observed in the employment of the Arnold sterilizer as to the heat applied. Should the 

 flame be fierce and the temperature employed very high, it is not safe to depend upon 

 the maintenance of the water-level in the reservoir; and should the water disappear, 

 the intricate joints of the apparatus may have the solder melted, especially about the 

 small feed tubes, the damage costing considerable inconvenience and expense in repair. 

 If used with ordinary care there is little danger of such accidents, but with a large flame, 

 as from a gas stove, the whole apparatus is apt to become overheated, and the condensa- 



FIG. 6. DIAGRAM OF ARNOLD STEAM 

 STERILIZER. 



