238 



DYEING. 



Of ScuA. three hours in common water, he found that its weight 



X "" P V"" I/ was diminished -.'unit one-eighth ; and upon repeating 

 the operation, the loss of weight amounted nearly to 

 one-fourth. The silk .-till retained yellowish or cha- 

 mois colour ; and though it did not possess a sufficient- 

 ly bright ground for the more brilliant colours, it an- 

 swered very well for those which were not affected by 

 the tinge it retained. It took a finer and more glossy 

 black, for example, than if it had been scoured in the 

 usual way with soap. 



Whitening 194. When silk is intended for the manufacture of 



of ulk with- 



ootiAct. 



ing iucUs- 



tifitjr. 



Preparation 

 ofnlkwith 

 alum. 



blonds and gauses, its natural elasticity and stillness 

 should be preserved as much as possible ; but as the 

 methods usually employee! tor removing the colouring 

 nutter, also deprived it of the gum, it became a desi- 

 deratum to whiten silk without affecting its elasticity. 

 M . lU-aume discovered a method of doing this by means 

 of muriatic acid and alcohol. The process, however, is 

 said to be liable to accidents ; and, notwithstanding the 

 advantages which it appears to present, it is scarcely 

 employed. 



1 !).". Before silk can be dyed, it must be prepared 

 with alum, as without this operation the greatest part 

 of the colours applied would possess neither beauty nor 

 fixity. This preparation consists in mixing, in a tun or 

 vat, about forty or fifty pails of water, with an equal 

 number of pounds of Roman alum, which has been 

 previously dissolved in warm water ; the solution being 

 well stirred during the mixture, to prevent the alum re- 

 turning to the crystallized state. The silk, after being 

 well washed and wrung, to free it completely from any 

 soap which may have adhered to it, is immersed in the 

 alum liquor, where it is allowed to remain for eight or 

 nine hours, after which it is wrung out, and washed 

 in a stream of pure water. The above quantity of li- 

 quor is sufficient for 150 Ibs. of silk, and with the ad- 

 dition of a little more alum when the solution begins 

 to grow weak, may be used for a fresh portion, as be- 

 fore : this addition may be repeated until the liquor ac- 

 quires a disagreeable smell, and it may then be cm- 

 ployed in the preparation of stuffs intended for dark co- 

 lours, till all its strength is exhausted. The prepara- 

 tion must be performed in the Cold, because the liquor, 

 when it is employed hot, impairs the lustre of the silk. 

 See SILK. 



CHAP. IV. Of Cotton. 



Different 196. Cotton is the downy substance contained in the 



kinds of pods of a genus of plants denominated Gossypium, of 

 which Linnaeus describes five species, the herbaceiim, 

 arborcum, hirsiitum, re/igiosnm, and barbadensc. The 

 qualities of the cotton vary in these different species, 

 and depend not less upon the climate than upon the 

 plant itself. Great varieties of the gossypium occur in 

 the American islands ; but, according to Mr Bennet, 

 the inhabitants have hitherto neglected to choose the 

 most valuable species, and have thus lost much of the 

 benefit which they might have derived from this valu- 

 able article of commerce. 



197. Cottons differ principally in the length of the 

 filaments, their fineness, strength, and colour. The 

 structure of the fibre has not been well ascertained. 

 Lewenhoek, who examined it with a microscope, affirms, 

 that it has two sharp sides; a conformation which serves 

 to explain the irritating property of cotton, when it is 

 employed in dressing wounds and ulcers instead of 

 lint. Dr Bancroft seems to ascribe the disposition which. 



cotton luu of imbibing colours more readily than linen <>i' smffV 

 to this mechanic:'.! structure; and yet, \\U\\ some de- S ~^V^^ 

 gree of inconsistency, he In i considerable 



pains in controverting a similar opinion of M. le 1'ileiir 

 d'Apligny. "This author," -u\ s lie, endeavoured 

 il iin the caiis-e why colours are less durable when 

 dyeil in silk, cotton, ami linen, than in wool, by sup- 

 posing that the pores of the three first of these sub- 

 stances were smaller than those of wool ; and that, 

 therefore, colouring particles could not enter into them 

 so easily and freely as into those of wool. But the very 

 r< ver.se of tliis supposition seems true, there being lit- 

 tle difficulty in making silk, cotton, or linen, imbibe 

 colours, even when topically applied cold, without any 

 artificial dilatation of their pores, which is necessary in 

 the dyeing of wool. The real difficulty, therefore, is 

 not in making them imbibe, but in making them retain 

 the colouring particles when imbibed ; because being 

 admitted so readily into their undilated jiores, they can- 

 not be afterwards compressed and held therein by any 

 contraction of these jiores, as is done in those of wool. 

 We know that it requires twice as much cochineal to pro- 

 duce a crimson on silk as on wool, which is a proof 

 that it can take up a greater quantity, and consequent- 

 ly that its pores are at least sufficiently large and ac- 

 cessible: we know also, that unbleached cotton is al- 

 ways preferred for dyeing the Turkey red, it being 

 found to retain the colour most permanently ; doubt- 

 less, because its pores or interstices are less open before 

 than after the operation of bleaching. This is also tin- 

 case of raw or unsecured silk, which, as the ingenious. 

 Mr Henry of Manchester observes, is more easily and 

 permanently dyed than that which has passed tin-above 

 described process of whitening and scouring . and, in- 

 deed, the openness of the pores of cotton and linen, 

 and their consequent readiness to imbibe both colour- 

 ing particles, and the earthy or metallic bases employed 

 to fix most of them, are circumstances ujion which the 

 art of calico-printing is in a great degree founded." 

 The sentiments of Dr Bancroft respecting the cause 

 of the union and durability of the colouring matter 

 with the stuff, as they are expressed in the passage we 

 have just quoted, seem to agree completely with the 

 exploded notions of Hellot on the same subject. Both 

 refer the operations of dyeing to the dilatation of the 

 pores of the stuff; to the deposition in them of particles 

 of foreign matter ; and to the retention of these par- 

 ticles in their new situation, by the subsequent contrac- 

 tion of the pores. No notice whatever is taken of the 

 influence of chemical affinity, but the whole proc< 

 described as depending upon mechanical principles. 

 This is the more surprising, as Dr Bancroft, in other 

 parts of his excellent treatise On the Philosophy of Per- 

 manent Colours, has distinctly referred the combination 

 of the colouring matter, with the stuff to which it is 

 applied, to the influence of chemical attraction. ^. 



1 98. In order to dispose cotton to receive the dye, it Process of 

 must undergo the operation of scouring. It is some- scouring, 

 times boiled in sour water, but more frequently in al- 

 kaline ley. The boiling is carried on for two hours, 



after which it is wrung out; it is afterwards well rinsed 

 in a stream of water, and then dried. Cotton stuffs 

 are prepared for calico printing by soaking them in wa- 

 ter mixed w ilh at most l-50th of sulphuric acid, wash- 

 ing them afterwards in a stream of water, and then 

 drying them. Berthollet observed, that the acid used 

 in this operation, combined with a quantity of calca- 

 reous earth and iron, which would injure the colours. 



199. To prepare cotton for being dyed, it must also be 



