PREPARATION OF CULTURE MEDIA. 15 
requisite quantity of broth (enough to fill the tube to 
the depth of an inch and a half or two inches) is 
allowed to run in; the clip is then re-applied and the 
tube removed and plugged. This process is repeated 
until enough tubes have been filled. 
The tubes and the broth which remains over (after 
having been poured back into the flask and the latter 
plugged with cotton-wool) are now sterilised. The 
vessels are placed in the steam steriliser and exposed to 
steam for half an hour on three successive days; this 
process is called intermittent sterilisation, and its 
rationale is very simple. The first steaming destroys 
all developed bacteria, and would sterilise the fluid 
entirely if no spores were present. In the interval 
between the first and second sterilisation most of the 
spores which may be present will develop into mature 
bacteria, and these will be killed by the second steam- 
ing. The third sterilisation is to kill off any bacteria 
which may not have developed from spores in the first 
interval. A very similar process is adopted by the 
gardener in freeing soil from weeds; the application of 
chemical weed-destroyers or a thorough hoeing will 
destroy developed plants but will not injure seeds 
which may be contained in the soil, and these processes 
are repeated, intervals being allowed to permit the 
development of the plants until they reach the stage in 
which they are vulnerable. 
To recapitulate; mix the ingredients and heat until 
they are dissolved, render slightly alkaline, boil half an 
hour, filter. Then place in sterilised flask or into 
test-tubes and sterilise in the steam steriliser for half an 
hour on three successive days. 
NuTRIENT GELATIN is broth which has been solidified 
by the addition of from ten to fifteen per cent. of gelatin; 
