322 



Dr. Thomas Thomson on 



[May 



cations are of the same class. But the fugitive character 

 of these colours is to be attributed rather to the unstable 

 nature of the colouring materials, than to the manner of 

 their application. Could the colouring matter of madder, 

 for example, be produced pure and unchanged in a state of 

 solution, there is little doubt that the chemical which might 

 be obtained from it would be a permanent colour. A state of 

 solution more or less complete is necessary to the adhesion 

 of these chemical colours to cloth. 



1. Chemical Black. 



This colour is produced in a variety of ways. Some years 

 ago it was almost universally a decoction of nutgalls, to 

 which, after it became cold, nitrated peroxide of iron was 

 added, previously thickened with flour. A paste was thus 

 produced, of a slate colour, which being printed on the cloth 

 and afterwards exposed to the air, became black. Now it 

 is considered better to use a salt of the protoxide of iron, 

 which, after being printed on the cloth, is exposed to the 

 air. The iron is gradually peroxidized, and the colour be- 

 comes black. In a dry atmosphere, several days exposure 

 to the air are necessary to peroxidize the iron completely. 

 The black compound is then insoluble in water, and when 

 the cloth is washed, nothing is removed except the thick- 

 ening matter, and other superfluous matters which are 

 not combined with the fibres of the cotton. A chemical 

 black is also produced by mixing a decoction of logwood 

 with a protosalt of iron ; but it is not so permanent as the 

 gall black. 



