90 PHILOSOPHY OF ZOOLOGY. 



It is owing to the asperities of the surface of hair, that 

 the spinning of wool is so difficult. This is, in a great mea- 

 sure, removed, by besmearing it with oil, by which the in- 

 equalities are filled up ; or, at least, the asperities become 

 less sensible. When the wool is made into cloth, it is ne- 

 cessary to remove the oil, which is done by the process of 

 fulling. The cloth is placed in a trough, with water and 

 clay, and agitated for some time. The oil is removed by 

 the clay and water ; while the agitation, acting like pres- 

 sure, brings the hairs into closer union, and the cloth is 

 taken out, not only cleansed, but felted. The hairs of 

 every thread entwine themselves with those which are con- 

 tiguous ; so that the cloth may be cut without being sub- 

 ject to ravel. It is to this tendency to felt, that woollen 

 cloth, and stockings, increase in density, and contract in 

 dimensions, by being washed. In many places, woollen 

 stuffs are felted, on a small scale, by placing them in run- 

 ning water, or under cascades ; and the Zetlanders expose 

 them to the motions of the tides, in narrow inlets of the 

 sea. 



The colour of the peripheral tube exhibits very remark- 

 able differences. By some naturalists, the colour is consi- 

 dered as depending on the fluids contained in the pith ; 

 while, according to others, the seat of colour is in the horny 

 covering itself. The truth of the latter opinion, in certain 

 cases, at least, is obvious, by the inspection of the largest 

 hairs or spines of the porcupine, in which the pith is white, 

 while the horny covering is partly coloured. The sup- 

 porters of the former opinion contend, that the central part 

 alone is vascular, and endowed with vitality, as is demon- 

 strated in the disease termed Plica Polamca, in which the 

 hair bleeds, when cut ; that grief and anxiety have been 

 known to change the colour of the hair, by influencing, as 

 is supposed, the colouring secretions of the central vessels ; 



