NOTES ON METHODS, AND SELECTED REAGENTS. 493- 



the material can lie for twenty-four hours ; then be laid in distilled 

 water, frequently changed, for any time up to twenty-four hours. 

 If not required for use at once, instead of this, first wash in 50 per 

 cent, alcohol, then transfer to 70 per cent., then 80 per cent, and 

 finally to 95 per cent, alcohol, or strongest methylated spirit, 

 changing this after twenty-four hours if it is then discoloured. 

 In this they can be kept for use (see p. 445). 



Most algae are best fixed in 1 per cent, chromic acid, or in picric acid, 

 mixed with an equal volume of alcohol, for the same reason that 

 alcohol potash is often preferable to watery potash solution, viz.^ 

 to avoid undue swelling of the walls. 



SECTION-CUTTING. Various hints on this subject will be found scattered in 

 the text ; see "Sections" and "Embedding" in the General Index. 

 Where the material to be cut is alcohol-material, keep the razor and 

 surface wet with alcohol ; otherwise use water or, if the material i& 

 hard, glycerine. The razors are best hollow-ground ; but for cutting 

 wood use a razor only slightly hollowed, or flat on one side. Keep 

 the razors sharp. 



Thin objects which have to be cut can be placed in glycerine-gum, be- 

 tween pieces of elder-pith or smooth cork ; or even, if moderately 

 hard, between pieces of soft wood, such as poplar or lime-wood. 

 Very small objects may be embedded, either in celloidin or glycerine- 

 gum (see these headings in Index). For very minute object*, 

 such as pollen-grains, gum alone, or with a very small proportion 

 of glycerine, can be used. A layer of strong gum is placed on the 

 end of a piece of elder-pith, and when set nearly firm the pollen- 

 grains dropped on it, and then another drop of gum placed on the top. 

 For cutting with a microtome, see p. 444 et seq. 



Sections when cut may be lifted from the razor with a camel-hair 

 brush. If laid on flatly, they will not curl. With such a brush 

 a section can likewise be turned over. See p. 27. 



CLEARING. The cell-contents are often opaque ; and when it is desired 

 especially to see the cell-walls, as, e.g., in growing-points, the prepara- 

 tion must be cleared. Clearing reagents act by dissolving, or at 

 least swelling, the cell-contents. The best of these are : 



(16) Caustic Potash (5 or 6 per cent watery). 

 (16s) - Akoholic. 



(34) Chloral hydrate. 



(35) Eau de Javelle. 



After treatment with chloral hydrate or watery potash, wash in water ; 



after alcohol potash, in alcohol, and then mount in glycerine. 



Eau de Javelle is specially used for alcohol material. For method, 



see p. 210. After treatment, place in dilute alcoholic glycerine, 



and allow this slowly to concentrate. 

 Sections of alcohol-material which have been stained and are to be 



mounted in Canada balsam or dammar, must be cleared in another 



