GENERATION OF ANIMALS, II. n. 



may seem strange, because water is not thickened by 

 heat, vet semen is thick when it leaves the inside of 

 the animal, which is hot, and becomes fluid when it 

 cools. Moreover, watery substances freeze, but semen 

 does not freeze when exposed to frost in the open air ; 

 it becomes fluid, which suggests that it was heat that 

 thickened it. And yet it is not very probable that it 

 is thickened by heat, because it is substances that 

 contain a large proportion of earth which " set " and 

 thicken when boiled — milk, for example ; hence it 

 ought to soUdify when it cools, but in fact it does not 

 solidify at all ; the whole of it becomes fluid like water. 

 This then is the puzzle. Suppose that semen is water. 

 Water is never observed to be thickened by heat ; 

 whereas semen is both thick and hot, and the body 

 it comes from is hot. Or suppose it consists of earth, 

 or is a mixture of earth and water. In that case the 

 whole of it ought not to become fluid and turn to 

 water. Perhaps then after all we have not distin- 

 guished all the cases that occur. Other fluids thicken 

 beside those which are composed of water and earthy 

 matter, viz., those composed of water and pneuma,'^ 

 for instance, foam, which becomes thicker, and white ; 

 and the smaller and more microscopic the bubbles 

 are, the whiter and more compact is the appearance 

 of the bulk. Oil behaves in the same way ; it 

 thickens when it gets mixed with pneuma ; and that 

 is why (oil) when it becomes whiter is thickening, 

 since the watery substance in it is separated out from 



» Pneuma is defined below (736 a 1) as " hot air " ; see, 

 however, 736 b 35 ff . below. Rather than attempt a mis- 

 leading or inadequate translation of the word {e.g., spirit, 

 breath), I have decided to keep the original term, as else- 

 where. See further. Appendix B. 



159 



