GENERATION OF ANIMALS, V. iv. 



where the nature of the hair has but little heat and 

 the fluid which enters it is unduly plentiful, the heat 

 proper to the hair is unable to do its work and the 

 hair is putrefied by the heat present in the en\iron- 

 ment. All putrefaction, of course, is caused by heat, 

 but not by the innate heat. This has been stated 

 elsewhere." Water and earth and all such corporeal 

 bodies are liable to putrefaction, and therefore the 

 earthy vapour ** is liable to it as well : an example of 

 this is what is called mould : mould is in fact the 

 putrefaction of earthy vapour. So too the nourish- 

 ment in the hair, being of this kind, putrefies if it 

 does not get concocted, and what is called greyness 

 results. It is white, because mould too is white. 

 This is practically the only putrefied substance which 

 is white, and the reason for that is that it contains a 

 good deal of air : actually all earthy vapour is the 

 equivalent '^ of thick air. In fact, mould is as it were 

 the " opposite number " of hoar-frost, since if the 

 vapour which rises up gets congealed, hoar-frost is 

 the result ; if it gets putrefied, mould. And that is 

 why both occur on the surface, because vapour is on 

 the surface. So we see that the poets use a good 

 metaphor in their comedies when they jokingly call 

 white hairs the " mould " and " hoar-frost of age " : 

 one of them is generically, the other specifically, the 

 same as greyness : hoar-frost is the same generically 

 (both being vapour), mould is the same specifically 

 (both being putrefactions). Here is a sure sign that 

 this is its character : there are many instances of 

 people having grown grey hair as an aftermath of 

 disease, but later on when they were restored to 

 health dark hair took its place. The reason is that 



' For exet, ivvofuv cf. 780 b 9, and Introd. § 26. 



5S9 



