Fermentation and Kindred Phenomena. 91 



Beef tea, blood serum, infusions of turnip, cucumber, and other 

 vegetables, Pasteur's solution, &c, and occasionally solids such 

 as potatoes. But for the present we need only consider liquid 

 media. When freshly prepared they usually swarm with 

 organisms. This cannot be avoided; our only course is to 

 destroy them. To do this the liquids are placed in suitable 

 vessels (usually glass flasks or test tubes) the mouths of 

 which are plugged with cotton wool. They are then heated to 

 the temperature of boiling water, either by immersing them in 

 steam or by boiling their contents. The heating is usually 

 repeated on three consecutive days, so as to ensure the destruction 

 not only of the organisms originally present as such, but also 

 those which may have subsequently germinated from the more 

 resisting spores. The liquids are now sterile, and no organisms 

 can gain admission to them so long as the plug of cotton wool 

 remains undisturbed. To prove the sterility, the liquids ought 

 to be kept for some time, and should show no cloudiness or 

 other evidence of change. We have next to introduce an organ- 

 ism or spore of the particular species we wish to cultivate, free 

 from any others of a different species. 



To do this was, for some time, an impossibility, for how pick 

 out a single individual when to see it requires the highest powers 

 of the microscope ? We are indebted mainly to Koch for having 

 solved this problem, and for having devised a beautiful and 

 ingenious method which has marked quite an epoch in bacterio- 

 logical science. 



Koch takes a sterile nourishing medium containing gelatine, 



which when cold solidifies to a jelly.* He then introduces into 



the gently-warmed medium a droplet of liquid containing the 



organisms to be cultivated (but presumably other organisms also) 



and pours the mixture upon a glass plate which has been 



previously heated to a high temperature to destroy any organisms 



present in the dust on its surface. The plate is then put beneath 



a bell-shaped jar on blotting paper previously soaked in corrosive 



sublimate solution. By this means the jelly is kept moist, and 



at the same time protected from dust. 



* Probably everyone knows that ordinary jellies "set" on account of the gelatine 

 which they contain. 



