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Chapter I 

 DYES FIRST USED 



During the early investigations of dissected tissues, different 

 chemicals were applied in the hope of finding some means by which 

 the structural details of cells could be seen more clearly under a 

 lens. Frequently, mineral acids were tried; a tincture of iodine 

 was a common reagent. Iodine is still used in the examination of 

 temporary mounts where it colors proteins brown and starches 

 blue. The first organic colors tested as biological stains were the 

 mordant dyes extracted from insects, shellfish, roots, leaves, and 

 wood. The technique inherent in the application of these natural 

 dyes developed slowly because the need and action of mordants was 

 little understood. With the exception of Carmine and Haema- 

 toxylin the natural dyes have been replaced in microtechnique by 

 synthetic coal-tar dyes. 



Occasionally, coal-tar dyes which are important in a special 

 branch of commercial work are tried in histological procedures. 

 If the results prove satisfactory to the investigator, the dye may be 

 introduced as a special laboratory technique; then the dye is con- 

 sidered a biological stain. The biologist, however, can learn many 

 important facts from an examination of the technique which has 

 been continuously improved by industrial chemists. Large-scale 

 dyeing of wool, silk, and cotton has demonstrated the values and 

 limitations of dyes. Consequently, the manufacturer insists both 

 on a selection of a dye appropriate to each fabric and on a technique 

 that has proven universally effective. 



Coloring matters, according to their manner of application are 

 designated as acid, direct, sulfur, mordant, vat, or basic dyes. 

 Acid dyes are used on wool fabrics in a bath containing sulfuric, 

 acetic, or formic acid. Because of their ability to stain cotton 

 without an intervening agent, a few dyes are named direct dyes. 

 On cotton fabrics, sulfur dyes are resistant to the injurious action 

 of alkalies. Naturally occurring dyes require the aid of a mordant 

 including compounds of aluminum, antimony, chromium, iron, tin, 



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