DISEASES AND ENEMIES. 
551 
flask plugged with cotton wool, the active organisms 
are killed, and the air, returning through the cotton 
plug, as the steam condenses after removal from the 
Are, has the germs it may contain strained out of 
it. The milk would probably (not certainly, for a 
reason presently evident) remain good for an inde- 
finite period, and would be called “ sterile ” — i.e., 
devoid of germs. If a single Bacillus alvei could now 
be transferred to this milk, the bacillus would mul- 
tiply, and we should have a cultivation. But milk is 
opaque, contains fat-globules, and is in many ways un- 
suited to purposes of investigation ; we therefore proceed 
thus : As the cotton wool is likely to become a source 
of contamination, the germs in it are destroyed by 
placing it, in a tin box, in the kitchen oven, and 
giving it, for about an hour, heat enough to very 
faintly discolour it. Test tubes, about bin. by Jin., 
are, after careful washing, similarly “evened’’; and now 
the cotton wool is fitted into their necks, making 
plugs tight enough to hold well in, Jin. deep (/>, C, 
Fig. 122). The evening process it is now desirable to 
repeat ; but it may be omitted. A transparent, nutrient 
jelly is made by chopping finely Jib. lean and very 
fresh beef, soaking it in looz. of water twelve hours, 
and straining carefully, pressing the meat as dry as 
possible. Now add half a salt-spoonful of salt, and 
a bare ounce of the best transparent gelatine ;* then 
heat cautiously until the boiling point is reached 
(taking care that the gelatine is not burned). A 
little technical matter must here receive attention. 
* ^oz. of peptone {Peptonum siccum) would improve the medium, 
but may be omitted. 
