LABORATORY EXERCISES IN BACTERIOLOGY. 



30 



^t n t f eam '" WS n0t take place With certaint y. the tevel being quickly 



tost or all the water evaporated, after whieh the damage is done. With a proper flame 

 the reservoir once filled, the apparatus requires comparatively little attention- but 

 unless students are careful the appliance is likely to be more costly than the more simple 



In private work requiring steam sterilization it should be remembered that much 

 simpler apparatus than either of the forms mentioned may be extemporized whicn 

 accomphsh all that a reqmsrte, the expense attendant upon these set forms being en- 

 t,rely unnecessary. A cheap tin bucket, preferably of a tall shape, should be obtafned, 



several pieces of stone or brick placed in the 

 bottom, and a piece of metal, perforated in any 

 convenient manner, arranged upon these as 

 supports, thus forming a raised false bottom. 

 Water is put into the bucket nearly to the level 

 of the false bottom, the articles to be sterilized 

 introduced, and the cover adjusted and heat 

 applied. Some little additional care is requisite 

 to see that the water is not evaporated before 

 completion of the process ; but otherwise such a 

 crude apparatus presents no disadvantages and 

 will serve the purposes demanded as well as 

 apparatus costing a number of dollars. 



(c) For rapid and thorough sterilization an 

 autoclave or steam digester is commonly em- 

 ployed. In principle it is merely a heavy 

 metallic boiler, capable of withstanding con- 

 siderable steam pressure, fitted with an equally 

 strong cover with tight fittings and clamps for 

 close application to the boiler. There is usually 

 a steam gauge fitted into the coyer, and a 

 closed tube sunk in the cover for the reception 

 of a thermometer; a safety valve, set to what- 

 ever pressure is desired, is provided in the cover 

 or elsewhere; and generally, for convenience, a 

 small pet valve, closed by a screw plug, is also 

 arranged. In order to conserve the heat of the 

 flame the boiler is usually set upon a closed stand 

 of heavy sheet iron, the inclosed flame not being 

 deviated by draughts, and little heat lost by 

 The apparatus can readily be understood by reference to the diagram of 

 a section (Fig. 7). In the interior there is a false bottom or gauze bucket or other con- 

 :amer for the reception of such articles as are to be subjected to the process There 

 should be but little water put into the boiler (the bottom being covered with about an 

 >f water), and a strong flame applied. At first the cover should be but loosely 

 applied and the pet valve opened. When steam begins to escape, the articles to be 

 ihzed are to be placed in the boiler, the cover clamped in position, the pet valve 

 ed, and the time noted. Usually, an exposure of fifteen minutes is employed after 

 the temperature has risen to 120 C., the pressure at the same time registered by the 

 steam gauge being about thirty pounds to the square inch. The heat is then with- 



FIG 



7. SECTIONAL VIEW OF AUTO- 

 CLAVE. 



