52 . MICRO-ORGANISMS AND DISEASE [CH. IV 



from drying up, it is necessary to seal them up or to cap 

 them. The well-known indiarubber caps fulfil this purpose ; 

 unfortunately they are relatively expensive. Pouring a 

 layer of melted paraffin over the cotton-wool plug of the 

 mouth of the tube or flask is also used, but the process is 

 not a clean one, and this is specially felt when it is intended 

 to re-open the culture. I have for some years used, instead, 

 a method of sealing up which is not only reliable, but is 

 clean and very cheap, and easily renewed. I use gutta- 

 percha paper, of which a whole sheet only costs a few pence, 

 and is sufficient for several dozens of tubes ; circular pieces 

 are cut out of the sheet sufficiently large to cover the mouth 

 and neck of the culture-tubes ; the mouth and neck, after 

 burning in the flame the upper part of the plug, are slightly 

 warmed, the circular piece is neatly placed over it and the 

 outer part of the piece pressed on to the glass of the neck, 

 if necessary warmed so as to stick well, with the pressure of 

 the finger a complete air-tight closure can be effected. It 

 can again, when it is required to re-open the culture, be 

 easily pulled off and then replaced by a new gutta-percha 

 cap. As stated above this mode of closure is easy, cheap, 

 and neat. 



