68 BIOLOGY AND TECHNIQUE 



V 



tance, as v. Esmarch 1 has pointed out, that the steam shall be saturated, 

 that is, shall contain as much vaporized water as its temperature per- 

 mits. Unsaturated, or so-called " super-heated steam " is formed when 

 heat is applied to steam, either by passage through heated piping or over 

 heated metal plates. In such cases the temperature of the steam is 

 raised, but no further water-vapor being supplied, the steam exerts 

 less pressure and contains less water in proportion to its volume than 

 saturated steam of an equal temperature. The super-heated steam, 

 therefore, is heated considerably over its condensation temperature and 

 becomes literally dried. In consequence, its action is more comparable 

 to hot air than to saturated steam, and up to a certain temperature its 

 disinfecting power is actually less than that of live steam at 100 C. 

 v. Esmarch, who has made a thorough study of these conditions, con- 

 cludes that up to 125 C., the efficiency of superheated steam is lower 

 than that of live steam at 100 C. Above this temperature, of course, 

 it is again active as in the case of ordinary dry heat. 



PRACTICAL METHODS OF HEAT STERILIZATION. Burning. For ob- 

 jects without value, actual burning in a furnace is a certain and easily 

 applicable method of sterilization. Flaming, by passage through a 

 Bunsen or an alcohol flame, is the method in use for the sterilization of 

 platinum needles, coverslips, or other small objects which are used in 

 handling bacteria in the laboratory. 



Hot air sterilization is carried out in the so-called "hot air chambers," 

 simple devices of varied construction. The apparatus most commonly 

 used (Fig. 8) consists of a sheet-iron, double-walled chamber, the 

 joints of which, instead of being soldered, are closed by rivets. The inner 

 case of this chamber is entirely closed except for an opening in the top 

 through which a thermometer may be introduced, while the outer has a 

 large opening at the bottom and two smaller ones at the top. A gas- 

 burner is adjusted under this so as to play directly upon the bottom of 

 the inner case. A thermometer is fitted in the top in such a way that it 

 penetrates into the inner chamber. The air in the chamber is heated 

 directly by the flame and by the hot air, which, rising from the flame, 

 courses upward within the jacket between the two cases and escapes at 

 the top. To insure absolute sterilization of objects in such a chamber, 

 the temperature should be kept between 150 and 160 C. for at least an 

 hour. In sterilizing combustible articles in such a chamber, it should be 

 remembered that cotton is browned at a temperature of 200 C. and 



1 v. Esmarch, Zeit. f. Hyg., iv, 1888. 



