122 'ESSAYS ON BACTERIOLOGY. 



is now poured upon a sterilized plate of glass, where it 

 quickly hardens into a transparent film resembling 

 gelatin. Placing this plate in the moist chamber, 

 and enclosing the whole in the incubator, we await 

 the resulting culture. After some days we find the 

 plate studded with bacterial colonies of various ap- 

 pearance. Ocular and microscopic examination of 

 these colonies reveals the fact that several varieties of 

 microbes were present in the throat and are growing 

 on the plate. Repeating the procedure, we make 

 plate cultures from the different colonies, each suc- 

 ceeding experiment affording more nearly a pure cul- 

 ture of one variety. Finally, we reach, upon differ- 

 ent plates, pure cultures of each variety of micro-or- 

 ganism which grew upon the first plate. One of 

 these we presume to be the germ of diphtheria. We 

 now proceed to test the presumption. Animals are 

 inoculated with each variety of bacterium, the one 

 which produces diphtheria in the animal being now, 

 with stronger couAdotion, believed to be the one sought 

 for. Carefully studying this germ and recording the 

 resiilts of the study, we go back and repeat a number 

 of times the whole procedure. We examine many 

 cases of diphtheria and identify this germ in all. We 

 thus prove its constant presence in the disease. We 

 re-cultivate it and repeat many times the successful 

 inoculations, finding it always present in the artifi- 

 cially-produced disease; and thiis we complete the 

 chain of proof and the identification of the germ of 



