DYEING. 



215 



improve- 

 ment of 

 dyeing. 



llUiory. he had also been deprived of a great part of his just 

 profits by various infringements of the act. A bill 

 for that purpose was accordingly passed in the House 

 of Commons, in the 39th of his present Majesty ; but 

 it was lost in the House of Lords, in consequence of 

 the strong opposition which it encountered from a 

 great number of persons in the northern part of the 

 kingdom, some of whom had grown rich by the dis- 

 covery. It does not appear that the public derived 

 much advantage from this alienation of the Doctor's 

 right, as the bark in question soon rose to three times 

 the price at which he had invariably supplied it, and 

 would have been bound to supply it, for another term 

 of seven years, if the bill had passed in his favour. 

 The policy of sanctioning a monopoly expressly by law 

 may be justly questioned ; but every candid and liberal 

 mind will readily admit, that, considering the very im- 

 portant improvements in dyeing which Dr Bancroft has 

 introduced, the long and severe labour to which he 

 li;i-i submitted, and the great expence which he has 

 incurred in the prosecution of his inquiries, he is en- 

 titled to some higher remuneration for his services, 

 than the country lias yet thought proper to bestow. 

 Concluding 3i. " Of all the arts," says Berthollet, " that of 

 remarks re- dyeing, perhaps, with respect to its theory, requires 

 ipecting the jj, e mos t extensive knowledge of natural philosophy ; 

 because it is that which presents the greatest number of 

 phenomena to analyze, of uncertain changes to ascer- 

 tain, and of relations to establish with air, light, heat, 

 and many other agents, of which our knowledge hi- 

 therto has been very imperfect." But these remarks, 

 though certainly well-founded, ought not to discou- 

 rage. If many facts remain still to be explained, science 

 has already done much in determining the essential cir- 

 cumstances of particular processes, and excluding such 

 as are either absurd or superfluous ; in analyzing the 

 substances employed by the dyer, and discovering the 

 causes to which their action is to be ascribed ; and last- 

 ly, in suggesting the means most likely to improve the 

 art. 



33. In the following account which we propose to 

 give of tin- operations of dyeing, our object is not so 

 much to bring forward a new treatise, as to collect and 

 condense every thing useful that has already been writ- 

 U-n on the subject ; and to present a view of the art, 

 which shall include the most recent improvements, with- 

 out neglecting the description of those processes which 

 have long received the sanction of experience. In the 

 execution of this plan, we shall freely avail ourselves of 

 tin- lalx>urs of the most approved writers on dyeing, 

 particularly of Berthollet and Bancroft, as well as of the 

 different individuals who may have suggested improve- 

 ments in particular processes. In short, we shall aim 

 ;:t iisefuh.f* rather than novelty, and never introduce 

 theoretical views, unless they have an obvious tendency 

 !.. -iniplify or improve practical operations. When facts 

 occur which we cannot explain, we must be satisfied with 

 detailing the process of the art, and wait for further 

 light from experience, before we attempt to reduce 

 them to theory. 



SECTION II. 

 OF COLOURING MATTERS. 



CHAP. I. 



Of Colouring Matters in general. 

 "C THE knowledge of the laws according to which 



bodies appear of various colours, by absorbing some of 

 the rays of light, and reflecting others, is of very little 

 use to the practical dyer. His object is merely to ex- 

 tract the colouring principle from the substances which 

 form its original basis; to transfer it to the stuff; to 

 modify it by those agents which have been found by 

 previous experiment to affect its nature ; and to give it 

 permanency in its new state of combination. Still, 

 however, it may not be altogether foreign to our pur- 

 pose to take a slight view of the theoretical opinion* 

 which are entertained on this subject. 



37. The property which bodies possess of uniformly 

 reflecting certain rays of light, constitutes their colour : 

 when all the rays are absorbed, a body appears totally 

 destitute of colour, or black ; and when particular rays 

 are absorbed, and others reflected, it seems to be of 

 the colour which the reflected ray is capable of exci- 

 ting in our organs of vision. When the substances 

 which thus appear coloured are transferred to colour- 

 less bodies, they generally communicate to these bodies 

 their particular colour ; and when they are transferred 

 to bodies already coloured, they blend or mingle their 

 colour with that of the bodies to which they are ap- 

 plied, and give rise to mixed colours. 



38. The aptitude of reflecting particular rays of light, 

 seems to depend not so much upon the chemical con- 

 stitution of a body, as upon a certain arrangement, or 

 disposition of the particles of the surface. Many bodies 

 display different colours, according to the particular 

 angle under which they are viewed ; while others as- 

 sume a change of colour, simply by the change of their 

 mechanical condition. 



39. Hence it appears that colour ought not to be re- 

 garded as a distinct principle, existing separately from 

 the coloured body ; but merely as a faculty which the 

 constituent elements of bodies possess of reflecting par- 

 ticular rays of light decomposed at their surface, and 

 which is capable of being modified by changes in the 

 mechanical structure, or in the proportion of the ele- 

 mentary parts. It is therefore impossible to predict, 

 a priori, the colour of a compound body, from a know- 

 ledge of the principles of which it is composed. Fre- 

 quently two colourless bodies form a coloured com- 

 pound by mixture ; and it often happens that two sub- 

 stances, each of which has a very deep colour, are ren- 

 dered entirely colourless when they are united toge- 

 ther. In short, we can only determine by observation 

 and experiment, the colours which shall result from 

 particular combinations of bodies. 



40. These views, however, do not altogether accord 

 with the opinions of several eminent chemists, particu- 

 larly with respect to colours of vegetable origin. Many 

 seem to think that a peculiar proximate principle exists 

 in vegetables, in winch their colour generally resides, 

 and to which they have given the name of colouring 

 mailer. " In this opinion," as Mr Murray justly ob- 

 serves, " there is a degree of obscurity and vagueness." 

 Colour being, as we have already stated, a gecomh'.ry 

 quality, which may reside in any principle, and being 

 often exhibited by principles of the most opposite 

 kind, there is no good reason for supposing that there 

 is any distinct principle to which it exclusively be- 

 longs. At the same time, it must be admitted that 

 there is apparently a similarity in chemical constitu- 

 tion between certain vegetable products, in which 

 colouring matters reside in the deepest intensity ; and 

 that these colouring matters may be detached from 

 their state of combination by the agency of various sol- 

 vents, and transferred to other subsfcuices with which 



Cause of 

 colour. 



Colour nt 

 a distinct 

 principle 

 residing in 

 coloured 

 bodies. 



Vegetable 

 colouring 

 matter. 



