CHAPTER XII. 



MOnNTINQ OP PBEMANBNT PEBPAEATIONS. 



If a preparation has been successful, it is often 

 desirable to preserve it. It must, however, be clearly 

 understood from the outset that permanent prepara- 

 tions are only, so to speak, mammies, and that a 

 study of them cannot replace the observation of living 

 bacteria. Even stained preparations examined in 

 water look very diflferent from permanent prepara- 

 tions mounted in Canada balsam. The cause of this 

 ,is that all the water has been abstracted from the 

 latter, and hence the bacteria have shrunk up. If 

 we compare such a preparation with one lying in 

 water, both of which were made with material from 

 the same cultivation, the difference in the size of the 

 bacteria is easily seen. On the other hand, as a rule, 

 their forms remain unchanged ; it is only when the 

 preparation is imperfect that distortions occur. 

 Moreover stained preparations can only be kept a 

 limited time ; the colours gradually fade, and after a 

 few years become so faint that although the bacteria 

 themselves are still to be seen, it is impossible to 



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