l6o DYEING 



The methyl group affects the colour, for rosaniline is a very 

 slightly bluer magenta than pararosaniline. Side-groups such as 

 this are called modifiers. 



The familiar dye called basic fuchsine is a mixture of pararosani- 

 line with rosaniline; some specimens contain another related dye 

 as well. Since pararosaniline is the simplest of the triarylmethane 

 dyes, it could with advantage have been used as a type in the 



NH2 NH2 



,CH3 



\ 

 \ 



Y 



NH2 ci- 



+ 



Rosaniline 



experiments described later in this book (p. 324); but whereas one 

 must make pararosaniline for oneself, basic fuchsine is readily 

 available in commerce, and for this reason it has been chosen 

 instead. Almost exactly the same results would be obtained if 

 pararosaniline had been used. 



Magenta is another name for basic fuchsine. Mere inspection of 

 the solution suggests that it transmits much red light and some 

 blue, but absorbs light of intermediate wave-length. For accurate 

 comparison of the colours of dyes it is necessary to obtain figures 

 that will show the transmission or absorption at various wave- 

 lengths. This is done by use of a spectrophotometer. Light of 

 known wave-length is shone through a solution of the dye con- 

 tained in a flat-sided vessel of uniform thickness, and the intensity 

 of the light that comes through is measured by means of a photo- 

 electric cell. A measurement can also be made with exactly the 

 same apparatus and the same solvent (water, for instance), in the 

 absence of the dye. The amount of light that comes through when 

 the dye is present with the solvent can then be expressed as a 

 percentage of the amount that comes through when it is not. This 

 percentage is known as the transmission at the particular wave- 

 length used. It is usual to find the transmission at 10 mjx intervals 

 throughout the visible spectrum. A curve like that shown in fig. 17 

 is then obtained. It is generally best to dissolve the dye at a con- 



