OUTSTANDING BIOLOGICAL ADVANCES 33 



so incisive in analysis, that from time to time he was 

 called on by different countries to assist in the in- 

 vestigation of the causes and means of control of 

 infectious diseases such as Asiatic cholera, the 

 bubonic plague and the sleeping sickness of Africa. 

 Koch was intimately concerned in establishing 

 bacteriology as an independent science. In 1882, he 

 introduced into bacteriological study the ingenious 

 method of employing solid culture media. Bacteria 

 are so minute that it was a perplexing problem to 

 separate the several forms that are commonly mixed 

 and to secure a pure culture of one kind. This was 

 made possible by placing a liquid culture of the mixed 

 bacteria in melted gelatine and other media ren- 

 dered liquid by heat. This mixture was stirred with a 

 glass rod and by this means the bacteria were sep- 

 arated and distributed. The melted gelatine was 

 now allowed to flow over a sterilized glass plate, 

 making a thin film which hardened on cooling. In 

 this manner the separated bacteria were held in place 

 and as they grew in clusters, there were spots of pure 

 or nearly pure culture. These spots were observed 

 under the microscope, were picked off and planted 

 again in liquid gelatine and the process repeated 

 until pure cultures were secured. 



