GELATIN AND POTATO MEDIA. 163 



form a very fine suspension, can be sterilized in two to three hours. 

 Delay in the germination of spores, because of low temperature, or 

 because of shelter, may give rise to a contamination of the medium. 



In very warm weather, as in mid-summer, failure to obtain per- 

 fect sterilization may be due to another cause. The spores that may 

 be present, after the first heating-, will g-erminate but the young- cells, 

 owing to the prevailing high temperature, multiply rapidly and 

 eventually form new spores. Consequently, .there may be as many 

 spores present after the second heat, as after the first. Under these 

 conditions, the medium can be steamed on each of six or eight days, 

 and yet not become sterile. Sterilization, however, can be accom- 

 plished in such a case by steaming at intervals of 12 to 15 hours, or by 

 keeping the material at a low temperature (18-20°). 



Failure to secure sterilization may, in exceptional cases, as 

 pointed out by Smith, be due to the presence of spores of anaerobic 

 bacteria. Such spores are unable to germinate in the presence of 

 air, and consequently may survive fractional sterilization. The me- 

 dium will be apparently sterile, and it is only when aerobic bacterial 

 are planted, that the spores of the anaerobe are enabled to ger- 

 minate. 



Some substances cannot be heated to 100° without profound 

 change. Blood-serum, for instance, will coagulate when heated 

 above 70°. The principle of fractional sterilization, however, can 

 be applied to serum. Since the temperature cannot exceed even 60°, 

 it must be allowed to act for at least one hour, on each day. This 

 temperature is sufficient to kill most of the vegetating forms. The 

 spore resists perfectly, and it is only after germination that the 

 organism can be destroyed. 



The steam sterilizer. — The apparatus usually employed 

 in Germany, is known as the Koch steam sterilizer. It is 

 essentially a large cylinder, the lower part of which con- 

 tains water. A grating-, at about one-fourth the distance 

 from the bottom, serves to support the objects to be steril- 

 ized. The water is raised to boiling by means of a large 

 burner, and the steam escapes from the opening in the lid. 

 Owing to the large amount of water to be heated, much 

 time is lost in waiting for the appearance of active steam. 

 If, perchance, the gas pressure is low, it may be very 

 difficult to obtain any steam. In this country, the Arnold, 

 steam sterilizer is used extensively. 



