48 PRACTICAL BACTERIOLOGY. 



tubes with thick cotton-wool plugs, and must be sub- 

 jected to the fractional sterilisation. The plugs are, 

 as before, singed, and then covered over with india- 

 rubber caps, which have been sterilised in corrosive 

 sublimate solution. Then when the gelatine is re- 

 quired in the test-tubes, it is liquefied at a tempera- 

 ture of about 40° C, poured off in the manner de- 

 scribed above into the tubes, after which it is again 

 subjected to the fractional sterilisation. 



If the operation has been successfully accomplished, 

 the gelatine in the test-tubes is almost colourless, 

 perfectly clear and transparent, and only to be lique- 

 fied at a temperature of 25° C. ; if it should become 

 liquid at a lower temperature, it has, through some 

 accident, been kept boiling too long, and has thus 

 lost some of its power of solidification. 



The agar-agar mixture can be treated in the same 

 way, only as it can be kept boiling from one and a 

 half to two hours without its power of solidification 

 being affected, its sterilisation can be completed on 

 one day. After this has taken place the test-tubes are 

 removed from the wire basket, and the greater number 

 of them are placed in a slanting position in order that 

 the agar-agar may solidify obliquely. Liquid agar-agar 

 solidifies at a temperature of about 40°, whereas the 

 liquefaction of solid agar-agar only takes place at 95°. 

 Nutrient agar-agar is less transparent than nutrient 

 gelatine when it is solid. When it is hot and liquid, 



