316 CHEMICAL DISCOVEKY AND INVENTION 



together with the arsenic, partly in the form of arsenic acid, but 

 chiefly as arsenious acid. 



Magenta is a compound in which a base, called rosaniline, is 

 united with an acid. The product of the operations just described 

 is rosaniline hydrochloride, but in fact the rosaniline produced 

 consists of a mixture of two compounds, one of which, 

 C 19 H 17 N 3 HC1, is called para rosaniline, while the rosaniline is a 

 homologous compound containing C 20 H 19 N 3 HC1. These are 

 both red colouring matters which dye silk and wool direct, and 

 both are present together in the common dye. 



Another process for making aniline red or magenta avoids the 

 use of arsenic and yields a somewhat larger quantity of the 

 colouring matter. In this method the aniline and toluidine are 

 first converted into their solid salts by dissolving in hydrochloric 

 acid and drying. To this mixture is then added a further 

 quantity of the aniline oil together with nitrobenzene which is 

 the source of the oxygen required, and the whole is heated till 

 the temperature reaches about 190 C., when a small amount of 

 iron borings are added. The action is complicated. Other 

 processes have been introduced which, however, it is not neces- 

 sary to pursue further. 



Strangely enough the rosanilines alone are quite colourless 

 compounds and only form dyes when united with an acid, in the 

 proportions indicated in the formulae already given. Both these 

 bases are capable of combining with three molecules of hydro- 

 chloric acid, altogether forming yellowish brown crystallisable 

 compounds, but these again are not colouring matters. 



Magenta is easily soluble in water, forming a magnificent red 

 liquid from which large crystals, having the appearance of green 

 beetle wings, may be obtained by slow deposition. 



The use and appearance of substances of this kind have become 

 so familiar that few people of the present generation can imagine 

 the excitement and interest aroused when large frames covered 

 with jewel-like crystals of these dyes were shown in the Inter- 

 national Exhibition of 1862 in London. The crowds attracted 

 to the showcases containing these objects were as great as those 

 which gathered round the Koh-i-noor in 1851. 



If we refer to Hofmann's interesting Keport on the Chemical 

 Products and Processes in the Exhibition of 1862 a few facts 

 may be extracted which serve to show how strangely the con- 

 dition and distribution of the industry relating to colours from 



