DEW. 359 



" Vapour is a mixture," he said, " of water and heat, and as 

 long as water can get a supply of heat, vapour rises. But 

 vapour cannot rise high, or the heat would get detached 

 from it; and vapour cannot exist in windy weather, but 

 becomes dissipated. Hence, in high places, and in windy 

 weather, dew cannot be formed for want of vapour." He 

 derided the notion that the stars and moon cause the pre- 

 cipitation of dew. " On the contrary, the sun," he said, 

 " is the cause ; since its heat raises the vapour, from which 

 the dew is formed when that heat is no longer present to 

 keep up the vapour." 



Amidst much that is false, there is here a good deal that 

 is sound. The notion that heat is some substance which 

 floats up the vapour, and may become detached from it in 

 high or windy places, is of course incorrect So also is the 

 supposition that the dew is produced by the fall of con- 

 densed vapour as the heat passes away. Nor is it correct 

 to say that the absence of the sun causes the condensation 

 of vapour, since, as we shall presently see, the cold which 

 causes the deposition of dew results from more than the 

 mere absence of the sun. But, in pointing out that the 

 discharge of vapour from the air, owing to loss of heat, is 

 the true cause of the deposition of dew, Aristotle expressed 

 an important truth. It was when he attempted to account 

 for the discharge that he failed. It will be observed, also, 

 that his explanation does not account for the observed fact 

 that dew is only formed in clear weather. 



Aristotle's views did not find acceptance among the 

 Greeks or Romans ; they preferred to look on the moon, 

 stars, and planets as the agents which cause the deposition 

 of dew. " This notion," says a modern author, " was too 

 beautiful for a Greek to give up, and the Romans could not 

 do better than follow the example of their masters." 



In the middle ages, despite the credit attached to 

 Aristotle's name, those who cultivated the physical sciences 

 were unwilling to accept his views ; for the alchemists (who 

 alone may be said to have been students of nature) founded 



