306 REAGENTS AND PROCESSES 



one coverglass, and these should come to lie over each other 

 in putting on the coverglass, they may be properly arranged 

 without attempting to remove the coverglass (which usually 

 makes the matter worse) by heating the slide until the gelatine 

 becomes quite soft, and then drawing a hair under the coverglass, 

 with which the sections may be manipulated. It is sometimes 

 a difficult matter to put just the right amount of the gelatine on 

 the slip. To overcome this difficulty, heat the gelatine and pour 

 it out in a thin film over a clean glass plate. When it has become 

 cool, strip it from the glass; then cut off small squares of different 

 size, melt them separately on glass slips, and cover with the 

 coverglasses of the size to be used with subsequent preparations. 

 The film of gelatine should then be cut into wafers of the size 

 found to exactly fill out the space under, the coverglass. These 

 wafers should be kept from drying too much and free from dust 

 in tightly-stoppered bottles. 



Glycerine Gum. Dissolve 10 gm. of powdered gum arabic 

 in 10 c.c. of water and add about 40 drops of glycerine. This 

 is useful for imbedding hard seeds, pollen grains, etc., prepara- 

 tory to sectioning them. The wood tissues of hard seeds may be 

 softened as described on page 267. Put a drop of the glyc- 

 erine gum on a suitable pine block and submerge the material 

 in it, and leave the preparation in the air to dry. Sections may 

 then be cut free-hand with a razor or on a microtome. Wash 

 out the gum from the sections in water. 



Glycerine-iodine. See under Iodine and Glycerine. 



Gram's Method. This method is specially recommended 



o 



for staining bacteria, either in coverglass preparations or in 

 sections. The sections are stained in a mixture of 100 c.c. of 

 aniline water (prepared by combining about 5 c.c. of aniline 

 with 95 c.c. of distilled water), and n c.c. of a concentrated 

 alcoholic solution of gentian violet or, better, methyl violet. 

 This is filtered, and 10 c.c. of absolute alcohol are added to it. 

 The preparation is taken from the stain, rinsed in alcohol, and 

 transferred to a solution of 2 parts of potassium iodide, and 

 i part of iodine in 300 parts of distilled water, where it remains 



