38 IXFECTION AND IMMUNITY 



experiments." That is, we are able to cultivate them 

 artificially in suitable media, and to study their mode 

 of 'development in the laboratory, quite independently 

 of the animals from which our " pure cultures " were 

 obtained in the first instance. The culture fluids 

 used are prepared from the flesh of various animals ; 

 and when to one of these a certain quantity of gela- 

 tine is added, we have a "solid culture medium," 

 upon the surface of which some of these germs will 

 grow most luxuriantly. To start such a " culture," it 

 is only necessary to transfer, with proper precautions, 

 a minute quantity of the infectious material to the 

 surface of our culture medium, or into a fluid which 

 has been found to be suitable for the growth of the 

 particular organism which we desire to cultivate. A 

 second culture is in the same way started from the 

 first, and so on indefinitely. 



Now it is evident that these pure cultures furnish 

 us a ready means for testing the power of various 

 chemical agents to destroy the vitality of known dis- 

 ease germs, as shown by their failure to grow in a 

 suitable culture medium after exposure for a given 

 time to a given percentage of the disinfectant. Very 

 many experiments of this nature have been made. 

 We may say here, that the experimental data on 

 record indicate that those agents which are efficient 

 for the destruction of any one of the pathogenic 



