12 MICRO-ORGANISMS IN WATER 



If the gelatine is not required for immediate use, or 

 if after filling tubes some remains in tlie flask, the 

 cotton-wool stopper must be securely replaced and the 

 flask sterilised again by heating in the steam steriliser 

 on three successive days in the manner previously 

 described. An india-rubber cap should then be drawn 

 over the cotton-wool stopper and the flask placed in a 

 dark cupboard and protected from dust as much as 

 possible. Before filling the tubes the gelatine must be 

 gently heated in the steriliser until it is liquid. 



The gelatine culture medium if properly made 

 should look perfectly clear and nearly colourless in the 

 test-tubes, and should only melt at a temperature of 

 25 C. ; if this occurs at a lower temperature, then it 

 shows that the gelatine has been overheated in the 

 preparation. 



Glycerin-gelatine peptone. It is of course possible 

 to vary the composition of this gelatine-peptone by the 

 addition of other substances. By adding 5-8 per cent. 



any quantitative value is to be attached to the colonies of micro- orga- 

 nisms developing on a gelatine plate, as in the examination of waters. 



Another point to which special attention must be given is that the 

 gelatine should be preserved in test-tubes only for as short a time as 

 possible, as owing to the large amount of evaporation going on the medium 

 rapidly becomes concentrated and its culture properties altered. This 

 concentration may, to a great extent, be prevented by covering the cotton- 

 wool stoppers with an india-rubber cap, but in laboratories where much 

 work is going on this is almost out of the question, and it is far more 

 convenient only to fill as many test-tubes as are required for rapid use. 



That the length of time a particular culture material has been pre- 

 pared before use may in some cases exert an important effect upon the 

 growth of micro-organisms lias been strikingly exhibited by Kitasato,* 

 who found that for the satisfactory development of the tetanus bacillus 

 in broth, the latter must only be used when quite freshly prepared, for 

 in broth, say a week old even, the bacillus will only grow slowly and 

 badly, and the toxic products elaborated were much weaker in their action 

 than when quite fresh broth was employed. 



* ' Experimentelle Untersuchimgeu iibe das Tetanusgift,' Zeitschrift f'Hr 

 Hygiene, vol. x. 1HD1, p. 2.SU. 



