103 



Morocco, is not, like those points * covered with 

 perpetual snows. The Piton, or Sugar Loaf, 

 which terminates the peak, no doubt reflects a 

 great quantity of light, on account of the whitish 

 colour of the pumice stone thrown up by the 

 crater ; but the height of this little truncated 

 cone does not form a twenty-secondth part of 

 the total elevation. The flanks of the volcano 

 are covered either with blocks of black and sco- 

 rified lava, or with a luxuriant vegetation, the 

 masses of which reflect so much the less light, as 

 the leaves of the trees are separated from each 

 other by shadows of more considerable extent 

 than that of the part which is enlightened. 



Hence it results, that setting aside the Piton, 

 the peak of Teyde is in the class of those moun- 

 tains, which, according to the expression of Bou- 

 guer, are seen at considerable distances only in a 

 negative manner, because they intercept the light 

 which is transmitted to us from the extreme li- 

 mits of the atmosphere; and that we perceive their 

 existence only on account of the difference of 

 intensity, which subsists between the aerial light 

 which surrounds them,and that which is reflected 

 by the particles of air placed between the moun- 

 tains and the eye of the observer As we with- 



* According to Haest, and Jackson, Account of the Em- 

 pire of Morocco, p. 43. 



+ Traite d'Optique, p. 365 It follows from the experi- 

 ments of the same author, in order that this difference may 



