246 COLOURING PRINCIPLES. BLUE DYES. 



by the action of acids, first into a yellow and then into a red. 

 The red colour of many flowers possesses the same properties as 

 that of leaves when thus altered ; and this fact will possess a 

 higher degree of interest, when it is shown, as it hereafter will 

 be (Chap, xn.), that the leafy parts of flowers have the same 

 general structure as leaves, and often differ very little from them. 

 It is further probable that all the colours of flowers are caused by 

 the presence of chromule, altered by various chemical means ; in 

 all instances it may be seen, that these colours exist in the same 

 parts, namely, minute globules contained within the cells. It 

 has been observed that the colours of many flowers may be 

 greatly changed by cultivation ; in some species, as the Dahlia 

 and Poppy, great varieties occur without any obvious cause, 

 the seeds of the same parent, raised in the same soil, producing 

 flowers of extremely different hues ; whilst in other cases, the 

 hue is manifestly influenced to a great degree by the nature of 

 the soil. 



387. The colouring matter of rapidly- growing parts has 

 seldom sufficient permanence, when removed from the plant, to 

 render it valuable for the purpose of staining cloth, &c. ; and 

 the substances used for this purpose, in the arts of the dyer, 

 calico-printer, &c., are chiefly obtained from the heart-wood, 

 roots, or bark ; sometimes, however, from the softer parts, as the 

 leaves and fruit. The principal dyes, by the combination of 

 which all varieties of shade may be produced, are blue, violet, 

 red, yellow, fawn, and black ; and substances, yielding all these, 

 are produced in abundance by different tribes of plants. 



388. Of all the blue dyes, Indigo is the most important. This 

 is obtained from the juices of several different species of plants, of 

 which some grow in almost all parts of the torrid zone. These 

 plants are raised from seed, and are of very rapid growth, being 

 ready for cutting at the end of two months. A subsequent 

 growth from the same roots, is again ready for the sickle, in six 

 or eight weeks ; and more may be subsequently obtained. In 

 India, it i&not considered advantageous to obtain more than four 

 crops from the same seed, as the produce of each is less than 

 that of the preceding ; but in Egypt, the seed is sown only once 



