THE PREPAEATION OF CULTURE MEDIA 25 



ence has shown is specially suitable for the particular 

 orgauism which it is desired to obtain in an isolated 

 condition. When microscopic examination shows that 

 this organism has abundantly multiplied in the medium, 

 a minute quantity is transferred to a fresh portion of 

 the same medium, the growth or multiplication is 

 allowed to take place there, and a small portion is again 

 removed to a fresh quantit)^ of the medium. By repeal- 

 ing this transference a number of times, it is in some 

 cases possible to so purify the growth that finally only 

 one kind of micro-organism is present. The chance of 

 getting pure cultures in this manner is, however, so 

 uncertain that the method generally serves only as a 

 convenient means of preliminary purification. The 

 only reliable way of obtaining pure cultures by means 

 of liquid media is the dilution method^ an account of 

 which will be found on p. 28. 



The following are some of the principal liquid media 

 in use : — 



Beef Broth. — The liquid medium which is best 

 adapted for general cultivation purposes is beef broth 

 or bouillon to which an addition of 1 per cent, of pep- 

 tone has been made. This peptone-beef-broth is pre- 

 pared in precisely the same manner as has been already 

 described under gelatine-peptone, the omission of the 

 gelatine being the only difference (see p. 19). 



Milk. — Milk also affords a o-ood culture material, 

 and may be prepared by simply placing some in sterile 

 test-tubes and steaming it in the steriliser at 100° C. for 

 an hour on the first day, and from 20 to 30 minutes on 

 each of the two following days. By submitting it to 

 such a high temperature, the chemical composition of the 

 milk is altered, however, and in some experiments this 



would be undesirable. In order to sterilise milk with- 



§ 



out interfering with its chemical character it is heated 



