324 PHYSICAL SCIENCE 



In Chapters XVI. and XVII. the author indulges in one of 

 his favourite moralising episodes, suggested by the topics he has 

 been discussing in the previous pages. He takes the existence 

 of reflecting surfaces as his text, and from the calm surface of 

 still water passes on to artificial mirrors, contrasting the manners 

 and morals of early mankind, who had only pools and lakes in 

 which to see their faces, with the luxury and vice of later ages, 

 when the use of metals led to the invention of metallic mirrors. 

 In this retrospect, however, he places the discovery of the use 

 of iron before that of the other metals. The priority of bronze 

 and the reason for it are accurately stated by Lucretius : 



et prior aeris erat quam ferri cognitus usus, 

 quo facilis magis est natura et copia maior. 1 



BOOK II 



In this division of his work the author discusses various aspects 

 of the atmosphere and offers an explanation of the phenomena 

 which he describes. He distinguishes between the very bright 

 ether on high, and the moist, denser atmosphere which underlies 

 it, but thinks that they must pass insensibly into each other (66). 

 The atmosphere he regards as a continuous non-composite body, 

 capable of great range in tension, and forming the vehicle through 

 which the exhalations from the earth pass outwards to the sky. 

 It does not everywhere possess the same qualities. In its lower 

 parts next the earth it is dense and misty, owing to the terrestrial 

 exhalations, and is there warmed by the earth's breath, by the 

 reflection of the sun's rays from the ground, and from the fires, 

 artificial and subterranean, as well as from the warmth communi- 

 cated by living animals and plants, for life cannot exist without 

 heat. The highest portions of the atmosphere are exceedingly 

 dry, hot, and attenuated, owing to their nearness to the eternal 

 fires and the heat of the heavenly bodies. The middle parts, on 

 the contrary, are intermediate in character, but colder than what 

 lies above and below them (60, 61). It is the lower portions 

 that are subject to the greatest changes, for they receive the 

 earthly elements which involve such constant turmoil. The in- 

 stability of the air arises also in part from the motions of the 

 earth and from those of the sun, moon, and stars, to which cold, 

 rain, and other atmospheric disturbances are due (56, 61). 



Seneca, in passing on to discuss the nature and origin of 



1 De Rer. Nat. v. 1287. 



