232 



D Y E I N O. 



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Change* of 

 colour by 

 the disen- 

 gagement 

 of hydro- 

 gen. 



Theory of 

 Berthollet 

 on this 

 aubjcct. 



oxygen is set at liberty. Several of the oxides of the 

 metals suffer like changes by exposure to light ; the 

 oxides of gold and silver are in this way partially re- 

 vived and restored to the metallic state." Paper mois- 

 tened with a solution of the nitrate of silver is blacken- 

 ed in a few minutes, and the muriate of the same metal 

 undergoes that change still more speedily. 



163. The effects of litjht upon the colouring matter 

 of vegetables are equally striking, and more immedi- 

 ately connected with the subject under consideration. 

 These effects are also to be ascribed to the disengage- 

 ment of oxygen. In the process of bleaching, the en- 

 louring matter of the thread is exposed to the agency 

 t>f that principle by the decomposition of the water 

 which is applied to the stuff. The combination of oxy- 

 gen with the colouring matter l>eing a species of com- 

 bustion, the colour is gradually destroyed, and the 

 substance to be bleached at last rendered white. This 

 theory receives a happy illustration from the powerful 

 agency exerted by the oxymuriatic acid in the de- 

 struction of colour. It appears, then, that lijrht pro- 

 duces changes in colouring principles, rather by disen- 

 gaging oxygen than by its own immediate operation. 

 We shall therefore consider a little more particularly 

 the action of that principle on colouring matters. 



16+. Fourcroy has made several observations on the 

 effects of oxygen on colouring particles, which enable 

 us to form some opinion respecting the nature of the 

 changes which they .undergo. His experiments were 

 chiefly made with watery solutions of colouring matter 

 left exposed to the atmosphere, or subjected to a boil- 

 ing heat He observed, that, in consequence of the 

 action of the air, vegetable decoctions formed pellicles, 

 which lost their solubility, and underwent successive 

 changes of colour. He marked the gradations of co- 

 lour thus produced, and concluded from his observa- 

 tions, that oxygen entered into the composition of the 

 colouring particles ; that when it combined with them, 

 their shade was ehanged,and that the more theyreceived, 

 the more fixed their colour became. He therefore in- 

 ferred, that the best method of obtaining permanent 

 colours for painting, was to choose such as had been 

 exposed to the action of the oxymuriatic acid. 



1 65. It appears, that, in some cases, the absorption 

 of oxygen by the colouring matter produces a change 

 of colour, in consequence of the oxygen combining with 

 the hydrogen, and thus forming water ; so that the 

 change which takes place is rather to be ascribed to the 

 separation of hydrogen, than to the permanent union 

 of die oxygen with the colouring particles. Berthollet 

 observed, that the oxymuriatic acid exhibited different 

 phenomena with the colouring matter ; that sometimes 

 it discharged their colour and rendered them white ; 

 that most frequently it changed them to a yellow, fawn 

 colour, brown, or black, according to the intensity of 

 its action ; and that, when their colour appeared only 

 discharged or rendered white, heat, or a length of time, 

 was capable of rendering them yellow. He concluded, 

 that the effects of the oxymuriatic acid were similar to 

 those of combustion, occasioning a destruction of the 

 hydrogen, which, combining with the oxygen more 

 easily, and at a lower temperature than charcoal does, 

 leaves the latter predominant ; so that the natural colour 

 of charcoal is more or less blended with the original 

 colour. Oxygen has certainly an influence in many 

 other cases on the changes which take place in the co- 

 louring particles of vegetables. These particles are 

 furmed chiefly in the leaves, flowers, and inner bark of 

 rees ; by degrees they undergo a slight combustion, 



either from the action of the ntmoipheric nir which sur- 

 rounds them, or from that of the ;:ir which is carried 

 iitii nl.ir set of vessels into the internal parts of 

 vegetables. l!<-i.ic most trees contain lawn coloured 

 particles, inclining more or U-- to \ ellow, red, or brown, 

 which, in consequence of till- combustion, grow thick, 

 and arc at last thrown out of the \ascular fibres of the 

 bark. 



From what has Ix-en rtated, it appears, that 

 changes may be produced in the colours of bodies, ei- 

 ther by the separation of oxygen, or by the union of 

 that principle w-ith one of the elements of the colour- 

 ing matter. In the latter case, which applies chiefly 

 to vegetable substances, the oxygen combines with the 

 hydrogen ; the attraction of the colouring particles for 

 their basis is thus weakened, and they are easily car- 

 ried off by water. This effect takes place more or 

 rapidly, according to the nature of the colouring ]>ar- 

 ticles.'or rather, according to the properties which they 

 possess in the state of combination into which they 

 nave entered. Colouring substances, therefore, i 

 the action of the air the less they are disposed to unite 

 with oxygen, and thereby to surfer, more or less quick- 

 ly, a smaller or a greater degree of combustion. Light 

 promotes this effect, which, in many case--, is not pro- 

 duced without its assistance ; but the colouring matter, 

 in its separate state, is more liable to suffer this com- 

 bustion than when united to a substance such as alu- 

 mine, which may either defend it by its own power of 

 resisting combustion, or, by attracting it strongly, w 

 en its action on other substances, which is the duel' ef- 

 fect of mordants; and, fin.-.lly. this last compound ac- 

 quires still greater durability when it is capable of com- 

 bining intimately with the stuff. 



167. Dr Bancroft has stated several objections to this 

 theory, some of which appear to possess considerable 

 weight, while others are founded upon limited views of 

 the subject. Berthollet has stated, that, in consequence 

 of oxygen combining with the hydrogen of colouring 

 matter, the charcoal becomes predominant, and mani- 

 fests its presence by the brownish appearance which 

 the colouring matter exhibits. Dr Bancroft seems to 

 conceive, that this explanation implied, that charcoal 

 existed in vegetable matters in the same sfcUe in which 

 it is found in charred wood, and that it was naturally 

 of a black colour ; but this supposition is by no means 

 necessary for the truth of the theory. It is a certain fact, 

 that in "whatever way the carbonization of vegetable 

 matter is effected, the charcoal obtained is always of a 

 dark colour : this is the case, whether the vegetable 

 matter be subjected to combustion in close ves.-els, or 

 placed in a situation (underwater, for example) where 

 the volatile matters either make their escape, or unite 

 with the bodies with which they are in contact. The 

 charcoal thus obtained, may not have existed in the 

 very same slate in the vegetable substance from which 

 it lias been procured ; and, indeed, there is every rea- 

 son to conclude, that charcoal is a combination of oxy- 

 gen and pure carbon ; but, at any rate, it is enough for 

 M. Berthollet to state, in support of his opinion, that 

 in all cases when the volatile parts of vegetable matter 

 have made their escape, the carbonaceous part which 

 remains behind is black. \Ve conceive, therefore, that 

 Dr Bancroft has done nothing to disprove the correct- 

 ness of Berthollet's opinion, by simply maintaining, 

 that " he should never be convinced that these mat- 

 ters had naturally contained ready formed black char- 

 coal, and that the degradation of the faded or injured 

 colour, resulted from a greater manifestation, and pre- 



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