136 MORRIS LOEB 



upon cotton or linen. They are the most brilliant and lively 

 dyes, but are strongly affected by sunlight, and are conse- 

 quently less useful than some dyes of other classes. They are 

 generally manufactured by oxidizing processes at a com- 

 paratively high temperature, whereby two or three simpler 

 compounds are welded, as it were, into compounds of com- 

 plex molecular structure. Thus, in the manufacture of the 

 well-known magenta dye (a tri-amido-compound) approxi- 

 mately equal quantities of aniline, ortho-toluidine, and para- 

 toluidine are heated from 8 to 10 hours with arsenic oxide to 

 190 C., in large iron kettles. A very thick mass results, 

 which can be extracted with hot water, and the compound 

 thus obtained is found to be made up of molecular quantities 

 of aniline, ortho-toluidine, and para-toluidine chemically 

 combined. 



Rosolic acid and its derivatives are made by the condensa- 

 tion of various phenols, three phenols being condensed into 

 one compound of the rosolic acid group, just as the bases 

 are condensed into one compound of the rosaniline group. 

 The comparatively few dyes of this group give various shades 

 of red. The hydroxyl groups, and hence the acid character 

 of the phenols, remain unchanged in the products of conden- 

 sation; the latter therefore combine with bases, and then they 

 readily go into solution. 



The phthaleins differ from the rosolic acids in so far as 

 one of the three phenyls of the triphenyl-carbinol is connected 

 in them with a carboxyl group (COOH), the other two 

 phenyls having one or more hydroxyls apiece, as in the rosolic 

 acids. The phthaleins were discovered by Adolph von Baeyer, 

 and are chiefly remarkable for the fluorescence of their alkali 

 salts in solution. They are prepared by heating phenols with 

 phthalic anhydride and a little sulphuric acid; when resorcin is 

 taken as the phenol, a very well-known compound is obtained, 



