16 W. H. E M I G 



taken from a fixative containing the salt of a heavy metal, mercuric 

 chloride, chromic acid, or potassium dichromate. The direct mor- 

 dant dyes have a more extensive application because they are not 

 adversely afTected by fixatives. Sections of paraffin materials are 

 left in 1 per cent ferric alum or 5 per cent ammonium alum for 30 

 minutes to an hour. After rinsing in distilled water, the section is 

 covered with the stain. Some of these dyes are inactive until they 

 are heated to 50° C. The depth of color in a finished mount may 

 be estimated from the section in water ; dehydration in alcohol ex- 

 tracts very little of the color. The time required in a mordant dye 

 differs: (1) Gallein one to ten minutes, (2) Gallocyanine one min- 

 ute to an hour, (3) Acid Alizarine Green G five minutes to an hour, 

 (4) Alizarine Cyanine G one minute to ten hours, (5) Carmine 30 

 minutes to 24 hours, (6) alum Haematoxylin 15 to 30 minutes, and 

 (7) iron Haematoxylin one to three minutes or sometimes as long 

 as 24 hours. 



Vat dyes, as a class, are very fast to light. Their application is 

 limited, at present, to the staining of blue-green algae, page 64. 



Basic dyes, Colour Index Numbers. 



680. Methyl Violet. 



740. Acridine Red. 



752. Rhodamine 6G. 



841. Safranine. 



924. Methylene Green. 



927. New Methylene Blue. 



All of these dyes are soluble in water. The stain should con- 

 tain 0.5 per cent of the dye, except Methyl Violet where only 0.1 

 per cent is needed. Acetic acid and ammonium alum are the 

 chemical assistants. 



The characteristic properties of basic dyes are derived from the 

 radicals -NH 2 , -N(CH 3 ) 2 , or -N(C 2 H )=- They are usually 

 marketed in the form of hydrochloride salts. In general, they 

 stain plant tissues 25 times more rapidly than the acid dyes. The 

 rate at which the staining process takes place depends on the extent 

 of ionization of the dye molecule. In the presence of a small 

 quantity of alkalies the color reaction is intensified, as in the stain- 

 ing of bacteria with alkaline Methylene Blue. The presence of 



