278 THE SOLIDS. 



A hair, however, which has been very forcibly extended will 

 not return to quite its original length, but will remain a 

 certain degree longer than it was previous to the extension. 

 A hair may be extended a third of its length without break- 

 ing ; elongated a fifth, it remains a seventeenth longer than 

 it was before ; it continues a tenth longer after having been 

 extended a fourth, and a sixth only after having been drawn 

 out as much as possible. 



Hairs, when they are dry, become electrical by rubbing 

 and emit sparks : this is well known with respect to the coat 

 of the cat, and Eble has observed the same thing to occur 

 in man. 



Hairs are also eminently hygroscopic, and imbibe moisture 

 from the air and from the skin, in consequence of which they 

 lose their set or curl, and become flaccid and straight. 



Nitrate of silver blackens the hair, a sulphuret of silver 

 being formed, and it is of this ingredient that the majority 

 of hair dyes are chiefly constituted. The concentrated 

 mineral aicds, especially the nitric, dissolve the hair, as does 

 also caustic potash. 



When heated hairs take fire and burn with a fuliginous 

 flame, emitting the odour of bone, and leaving a residue 

 of carbon. To dry distillation, they yield a quarter of their 

 weight of carbon, which is difficult to incinerate, the pro- 

 ducts being empyreumatic oil, water charged with ammonia, 

 and combustible gases, which comprise sulphureted hydrogen. 

 The ashes contain oxide of iron, traces of oxide of manganese 

 and silica, and of sulphate, phosphate, and carbonate of lime. 



THE HAIRS OF DIFFERENT ANIMALS. 



The precise structure of the hairs of different animals 

 varies considerably : those of the mammalia resemble the 

 hairs of man, or differ only in the degree of their develop- 

 ment, as the whiskers of the carnivora and rodents, and manes 

 and tails of horses, the bristles of pigs, &c. : it is in these 

 largely developed hairs that the structure can be best deter- 



