72 PHENOMENA OF HEAT AND SOUND. 



the nature of earth or air were considered to be hotter.* 

 Oil and also mercury were exceptional, because Aristotle 

 assumed that these contained much air,t while he considered 

 fat to be an essentially hot substance, because it readily 

 assumed the form of fire. I There is much uncertainty, 

 Aristotle says, about the nature of oil, for, whether it be 

 considered to be more of the nature of water or of earth, it 

 ought to be hardened either by the action of cold or by the 

 action of heat. It is not, however, hardened by either of 

 these, but merely thickened by both, the reason being that 

 oil is full of air.§ He does not state to what kind of oil he 

 refers, but it is probable that it was some kind of fish-oil, 

 which is not easily frozen. 



Long after Aristotle's time, philosophers held that there 

 was some body which, by its own nature, was particularly 

 cold, and that other bodies were cold in proportion to the 

 extent to which such essentially cold body entered into their 

 composition. According to Boyle, this body, well known to 

 philosophers as the Frimum Frigidiim, was considered by 

 some to be w^ater, by others earth, by others air, and by 

 some nitre, but he says "that water is the Frimum Frigidum, 

 the opinion of Aristotle has made it to be that of the schools, 

 and the generality of philosophers." || 



When classifying substances, partly by their composition 

 and partly by their behaviour under the action of heat, into 

 three classes which would now be called combustible with 

 evolution of much smoke, combustible without the evolution 

 of much smoke, and incombustible, Aristotle uses the well- 

 known term phlogistic, much employed before Lavoisier's 

 time. Aristotle gives pitch, oil, and wax as examples of 

 phlogistic substances, coal-like {anthrakeutic) substances as 

 examples of combustible bodies not yielding much smoke, 

 and bronze as an example of incombustible substances.^ 



It will be convenient to discuss next Aristotle's views 

 on the production of heat by friction. When expressing an 

 opinion that the heat and light of the heavenly bodies are due 

 to friction between them and the medium in contact with 

 them, as stated already in Chapter iii., he appeals to obser- 

 vations on the motions of darts and other missiles through 



* Meteorol. iv. c. 11, s. 7. f Ibid. iv. c. 8, s. 11. 



I P. A. ii. c. 2, 649a,. § Meteorol. iv. c. 7, ss. 2 and 8. 



II The Works of the Honoivrable Robert Boyle, new edition, London, 

 1772, vol. ii. pp. 585 and 591. 



II Meteorol, iv. c. 9, s. 37. 



