246 The Rev. A. Sedgwick's Outline of 



enables animal charcoal to decolorate similar liquids. Char- 

 coal, as we have also seen, withdraws metallic oxides from 

 their solution in alkalies. Cotton wool has the same power, 

 and it is extensively used as a means of dyeing with the yellow 

 and red chromates of lead. If lime in excess be added to 

 sugar of lead dissolved in a considerable quantity of water, 

 the lead, which precipitates is redissolved in the lime water 

 and forms a weak solution of plumbate of lime. If a piece of 

 cotton be immersed in this solution it appropriates the lead, 

 and when afterwards washed and dipped in a solution of 

 chrome, the lead becomes chromate of lead. 



The same force enables cotton to imbibe basic salts of iron 

 and tin by immersion in certain solutions of these metals; and 

 many other examples of what Berzelius calls a catalytic force, 

 in decomposing weak combinations, will occur to those who 

 are familiar with the art of dyeing. 



It appeared to me interesting to compare the amount of sur- 

 face exposed by cotton wool with that of the more minute di- 

 visions of charcoal. I am enabled to furnish the following 

 calculation through the kindness of Professor Balfour, who 

 has measured with great care the fibres of various qualities of 

 wool. The fibre of New Orleans wool varies most commonly 

 from ijoo tn to 2oVo tn 0,? an * ncn ni diameter. About forty 

 of these fibres or tubes compose a thread of No. 38 yarn 

 (thirty-eight hanks to the pound). Ordinary printing cloth 

 has, in the bleached state, 493 lineal feet of fibre, or 10*6 square 

 inches of external surface of fibre in a square inch, which 

 weighs nearly one grain. It is easy to compress 210 folds of 

 this cloth into the thickness of one inch. It has then a spe- 

 cific gravity of 0'8. One cubic inch has 94*163 lineal feet of 

 tube, and 16'8 feet of external surface; or, if we include the 

 internal surface, there are upwards of 30 square feet of surface 

 of fibre in one cubic inch of compressed calico. The char- 

 coal of box-wood has, as we have seen, 73 square feet of sur- 

 face to the inch, with a specific gravity of 0*6*. 



XXXVIII. Outline of the Geological Structure of North 



Wales. By the Rev. A. Sedgwick, F.G.S.f 



§ 1. Introduction. 



r r*HE author here describes in considerable detail the geographical 



-■• limits of the country under notice. For the structure of the Isle 



of Anglesea he refers to a paper by Prof. Henslow, published in the 



* For drawings of cotton and linen see Phil. Mag., Nov. 1834. (S. 3. vol. 

 v.p. 355.) 



•j- From the Proceedings of the Geological Society, vol. iv. p. 212, having 

 been read June 21, 1843. 



