PHENOMENA OF HEAT AND SOUND. 73 



the air. He says that darts had been seen to be heated to 

 such an extent that their leaden weights were melted, and 

 adds that the shock imparted by their rapid motion to the 

 air causes this to become fire.* This production of heat by 

 friction is referred to in several passages in his Meteorology, 

 where he tries to explain the phenomena of falling stars, 

 comets, and other fiery appearances, as described already in 

 Chapter iii. He was aware that the intensity of the heat 

 generated by a rapidly moving body was greater than that 

 generated by a body moving slowly. 



So far, he relies on the results of observation, but to a 

 large extent his ideas on the production of heat by friction 

 depended on his conceptions about the composition of 

 bodies from the four so-called elements. The facility with 

 which a substance becomes ignited by friction depends, 

 according to Aristotle, on the quantity of the element fire 

 or air, which was most nearly related to fire, in the substance 

 itself. He says that if pieces of wood, stone, or iron are 

 heated in consequence of their motion, it is still more likely 

 that air, which is most nearly related to fire, should be 

 heated. t 



In P. A. ii. c. 2, Aristotle discusses what is meant when 

 one substance is said to be hotter or colder than another, 

 and incidentally explains how a rough estimate of tempe- 

 rature may be made. It will be seen that he did not always 

 distinguish between temperature and quantity of heat. 

 This part of his discussion about heat is more than usually 

 interesting, and an epitome is therefore given in the follow- 

 ing paragraph. 



Some say that blood is hot and that bile is cold, while 

 others say that bile is hot and blood cold. If there is 

 this difference of opinion about heat and cold, which are 

 capable of producing distinct impressions, what is to be 

 thought of the impressions given by senses other than touch? 

 The difficulty may be explained by the fact that the term 

 "hotter" is used in several different senses, so that state- 

 ments apparently contradictory may all be more or less true. 

 In what senses then is the term "hot" employed? To 

 answer this question, it is necessary to find out what 

 particular effect is produced by a substance which is hotter 

 than another, or, if several effects are produced, to find out 

 how many such effects there are. In one sense, a body is 



- De Coelo, ii. c. 7, 289a. f Ibid. ii. c. 7, 289a. 



