STERILIZATION OF CULTURE MEDIA. 55 
uniformly, but should not be packed so closely that there is difficulty 
is removing them. 
Steam Sterilizers.—Steam at the ordinary pressure of the atmo- 
sphere has the same temperature as boiling water, and in practice is 
preferable to a water bath for several reasons. The form of steam 
sterilizer adopted by Koch, after extensive experiments made in col- 
laboration with Léffler and Gaffky, is now generally used in bacte- 
riological laboratories. This is shown in Fig. 24. It consists of a 
cylindrical vessel of zinc which is covered with a jacket of felt. 
The cover, also covered with non-conducting material, has an aper- 
ture at the top for the escape of steam. A glass tube, which is in 
communication with the interior of the vessel, serves to show the 
Hite 
Fie. 24. Fie. 25. 
height of the water when the apparatus is in use. The bottom of 
the cylindrical vessel should be of copper. A Bunsen burner having 
three jets will commonly be required to keep the water in ebullition 
and the upper part of the steam sterilizer filled with ‘live steam,” 
which should escape freely from the aperture in the cover to insure 
a temperature of 100° C. in the steam chamber. A perforated zinc 
or copper shelf in the interior of the cylinder serves to support the 
flasks, etc., which are to be sterilized. Usually they are lowered 
into the cylinder in a light wire basket, or tin pail with perforated 
bottom, of proper diameter to slip easily into the sterilizer. 
Fig. 25 is a sectional view of this sterilizer. 
The steam sterilizer shown in Fig. 26 ' is an American invention, 
1The Arnold steam sterilizer, manufactured at Rochester, N. Y. 
