Mr. W. Crum on a peculiar Fibre of Cotton. 337 



of holding all matters which are insoluble in water, that is, all 

 bodies which can be caught upon a filter, while the other is 

 possessed of no such enclosure. 



I take this opportunity, in reply to a review of my first 

 memoir on this subject, by M. Persoz, in his remarkable work 

 Traitt de V Impression des Tissus, of explaining that I attribute 

 to an attraction of surface those cases of dyeing where pure 

 cotton, by mere immersion, is enabled to decompose the solid 

 matters in solution, and to withdraw them from the solvent. 

 Such is the case with the solution of deoxidized indigo in 

 lime, with the plumbite of lime, with the various salts of tin, 

 and many other solutions. Cotton, as I have stated, acts in 

 these cases like charcoal and other porous bodies, and I have 

 seen no reason to confine the attraction in question to the 

 internal surface of the cotton fibre. 



But I have not ranked the aluminous mordant among the 

 class of bodies so attracted ; because cotton, when immersed 

 in a solution of acetate of alumina, has not the power of sepa- 

 rating its base. That solution must be applied to cotton and 

 dried in it; and then the alumina only atllieres, or loses the 

 power of being washed away, in proportion as the acetic acid 

 is removed by evaporation. I could see here no chemical 

 decomposition effected by the cotton wool, for the same salt 

 may be decomposed by evaporation in a glass vessel. In this 

 case I have represented the alumina as being held in the inte- 

 rior of the fibre, just as sand may be held in a bag whose in- 

 terstices are too narrow to allow its particles to pass. 



M. Persoz has remarked, however, that by evaporating a 

 solution of acetate of alumina in a glass vessel we do not so 

 thoroughly decompose it as by drying the same substance 

 upon calico. This I also have observed ; and although I have 

 been accustomed to ascribe the difference to the more exten- 

 sive division and exposure of the salt upon cotton, I have no 

 proof, and shall not deny, that the presence of cotton at a 

 particular stage of the evaporation may accelerate the decom- 

 position of the salt, and that its fibres may thus attract a portion 

 of alumina over their whole surface. If this modification of 

 the view I had given be correct, the action of the coton mart 

 proves at least that colouring matter adhering outside is not 

 so permanent as that which is held within the fibre of the 

 mature cotton. 



Neither view gives any countenance to the chemical theory. 

 Porous bodies are well known to attract, and even to decom- 

 pose, without chemically combining with the substances they 



Phil. Mag. S. 3. Vol. 35. No. 237. Nov. 1849. Z 



