16 PRACTICAL BOTANY 



side of the cover}, while a small piece of blotting-paper 

 is pushed up to the opposite edge of the cover-slip, 

 so that, when it comes in contact with the medium 

 in which the object is mounted, it will soak it up : the 

 space thus vacated by the medium is taken by the 

 reagent, and if the latter be supplied in sufficient 

 quantity a stream of it will pass under the cover-slip 

 and bathe the object. It is obvious that for such a 

 treatment to be successful the medium and the reagent 

 must be fluids which will mix. Students are warned 

 against too readily accepting negative evidence as the 

 result of observations by irrigation : the reagent may 

 frequently pass under the cover-slip without permeat- 

 ing the object, or the edges only of a section may be 

 affected : in order to insure the object being bathed by 

 the reagent, it is well to raise the cover-glass gently 

 with a needle, or even to raise the section itself slightly 

 with the point of a needle. 



8. Never use more than one cover-slip on a single slide, 

 though several objects may, if small enough, be covered 

 by one slip : the cover- slip should be as nearly as possible 

 in the middle of the slide. 



9. Pressure should never be laid on the cover-slip, except 

 in certain special cases : a bad section will not be im- 

 proved by being squeezed flat, while a good section may 

 be easily rendered worthless by such treatment. 



10. Before putting a slide aside for subsequent 

 observation be sure that the medium used is one ivhich 

 will not evaporate : alcohol, water, and solutions in water, 

 such as iodine solution, aniline sulphate, salt solution, 

 are all liable to evaporation, while chlor-zinc-iodine and 

 glycerine are not. 



