82 BIOLOGY AND TECHNIQUE 



minutes, or even instantaneously, in compressed steam at tempera- 

 tures ranging from 120 to 140 C. 29 



In all methods of steam sterilization, it is of great practical 

 importance, as v. Esmarch 30 lias pointed out, that the steam shall 

 be saturated, that is, shall contain as much vaporized water as its 

 temperature permits. 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 tem- 

 perature. 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, concludes 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 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 thermom- 

 eter 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 



28 Christen, Kef. Cent, f . Bakt., V, xiii, 1893. 

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



