52 MICRO-ORGANISMS IN WATER 



should be thoroughly rinsed with, distilled water by 

 means of a wash bottle, and is then turned preparation- 

 side downwards on to a clean slide, gently pressed with 

 blotting-paper, so that all moisture on the upper surface 

 is removed. It is now ready for examination with the 

 oil immersion lens, a drop of cedar oil being first placed 

 on the dried surface. This is only a very temporary 

 though rapid way of mounting preparations. If they 

 are required for permanent use as a reference, for ex- 

 ample they must be mounted in Canada balsam. In 

 this case the cover-glass must, after staining and wash- 

 ing, be allowed to become quite dry ; a small drop 

 of balsam is placed on the glass slide, and the cover- 

 glass is then, preparation-side downwards, deposited on 

 the centre of this drop of balsam, which spreads out, 

 and finally extends over the whole under-surface of 

 the cover-glass. After a few days the balsam has become 

 hard, and after a few weeks extremely hard at the edges. 

 This is called a permanent preparation, and the colour 

 will remain for a long time unchanged if it is preserved 

 in the dark. 



Staining of spores. If a preparation of, say, b. an- 

 thracis or b. subtilis containing bacilli and spores be 

 stained in the usual way with the ordinary aqueous solu- 

 tions of aniline colours, bright spots will be found, 

 sometimes in the middle or at one end of the stained 

 bacilli, or in isolated groups, which have not taken up 

 the colouring matter. These spots are the spores which, 

 as in the case of some refractory bacilli, the stain has 

 been unable to touch, but which by special treatment 

 may also be beautifully exhibited by coloration. 1 For 



1 Not all such unstained spots apparent sometimes in coloured pre- 

 parations are necessarily spores. Often when the culture is old and de- 

 generation of the bacilli has taken place, clear spaces which have not 

 taken up the stain are seen in the middle of the rod. Other causes may 

 also contribute to this appearance. 



