1825.] of Claude-Louis Berthollet. 89 



into the art of dyeing, there is one of so great importance, and 

 whose investigation at the same time involved such difficulties, 

 that it deserves particular notice. This is his mode of employ- 

 ing Prussian blue in the formation of the brightest permanent 

 blues and greens now in use. 



The remarkable brilliancy of this substance as a colouring 

 matter, early recommended it to the notice of chemists and 

 practical dyers, all of whom, however, were baffled in their 

 attempts to discover any means of fixing it in an equal and per- 

 manent manner upon cloth. Menon, Macquer, and Roland, 

 successively assayed this task, and each proposed a separate 

 process for performing it ; but in practice the same ill success 

 attended the proposals of each. In some, only a very pale 

 shade of colour was produced ; in others, where the colour was 

 at once deeper and moderately permanent, it was always found 

 unequally distributed over the cloth ; and in a third, the colour, 

 which at first was bright and equable enough, was at the same 

 time so fugitive, from its having been applied in a manner 

 merely mechanical, that a slight wearing speedily injured it, and 

 after a few washings it almost entirely disappeared. 



The mode in which Berthollet overcame all these difficulties 

 was a most ingenious one, and it was the result only of much 

 research and experiment on his part ; in the course of which he 

 was aided by the celebrated calico-printer Widmer. It occurred^ 

 to Berthollet that, as prussian blue is a compound substance, of 

 which one constituent by itself has a strong affinity for cloth, it 

 might be possible that the other constituent should unite readily 

 with the first, even on finding it previously combined with the 

 cloth, although the compound body so presented refuses any 

 such union. He, therefore, first treated cloth with oxide of 

 iron, one constituent oF prussian blue, for which the cloth has a 

 powerful affinity, and next superinduced upon the whole the acid 

 principle, by the application of an alkaline prussiate. The acid 

 colouring mutter, uniting with the oxide of iron, formed the dye, 

 without at all disengaging the previous combination between 

 the cloth and the oxide. The prussian blue communicated in 

 this manner is found to have nearly as strong an affinity for cloth 

 as the oxide of iron has when in separate combination with it ; 

 and thus, by the result of this method, a blue colour of the 

 greatest brilliancy and permanency was added to the art of 

 dyeing. 



In employing the same substance as an ingredient to produce 

 a green, M.' BerlhoUet's ingenuity was again severely tried, and 

 again it overcame every difficulty. To achieve this, there is one 

 other stej) necessary m the series of affinities, which are all 

 brought into play only by the onkv in which they are made to 

 follow each other; a process which in the end produces a com- 



