ROSE BENGAL C. I. NO. 777 OF 779 



Various shades denoted: Rose bengal B, 2B, 3B. 



{An acid dye; absorption maximum of C. I. No. 779 about 5^8.) 



This is a second derivative of di-chlor or tetra-chlor fluorescein, 

 and corresponds to erythrosin, as it contains four iodine atoms. As 

 in the case of phloxine, there are two similar dyes, the di-chlor 

 compound (So. Ill) and the tetra-chlor compound (Xo. 779); it 

 is not certain which is desired in microscopic work. The former 

 has the formula 



NaO 



O 



C 



_0 



I I 



ci_/\_ 



COONa 



•CI 



This dye has a pleasing deep pink color; and altho an acid dye 

 it proves to have considerable affinity for bacterial protoplasm, 

 if used in carbolic acid solution, and to have good selective prop- 

 erties when used as a bacterial stain. It has recently been recom- 

 mended by the author for staining bacteria in soil suspensions. It 

 has also been used as a cytoplasm stain following haematoxylin. 



4. PHEXOLPHTHALEIN AND THE SULPHOXPHTHALEINS 

 A phthalein is a compound of phthalic acid : 



COOH 



I 



COOH 



or rather of phthalic anhydride: 



CO 



CO 



o 



with phenol or a phenol derivative. If phthalic acid is heated 

 with phenol and sulfuric acid it combines with two molecules of 

 the latter and forms phenolphthalein. In the same way, a sul- 

 phonphthalein is a compound of ortho-sulpho-benzoic acid: 



SO3H 



COOH 



83 



