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 ito cloth, it is ne- 
cessary to remove the ol, 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 placmg 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 m 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 opmion contend, that the central part 
alone is vascular, and endowed with vitality, as is demon- 
strated in the disease termed Plica Polonica, im 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 ; 
