NOME VII. VOLCANIC IXTRITE. 491 



future to distinguish it by the name of volcanic Volcanic sand. 

 sand, one which already begins to be common. 

 On examining it with the microscope, this sub- 

 stance is seen to be composed of particles of a 

 rough and earthy appearance, mingled with tri- 

 turated fragments of felspar and augite. All are 

 not alike, some being of large and others of 

 smaller size. The grains are often of a dark 

 grey colour, inclining to black ; sometimes, and 

 especially on the last days they fell, they were 

 of a brighter ash-colour. It is constantly ob- 

 served that, when the volcanic sand that falls is 

 of a whitish colour, the eruption is near its end. 

 This white colour of the volcanic sand may be 

 derived from two causes; a greater trituration 

 and tenuity, as in the instance of green glass, 

 which when finely pulverised becomes white, or 

 a longer exposure to the action of acid vapours. 

 The sand ejected by the volcano, in the earlier 

 stages of its eruption, issues from a furnace full 

 of matter; but the vapours, as it begins to empty, 

 have room to act with greater effect on the re- 

 maining substances. Some particles of this vol- 

 canic sand placed over fire effuse a perceptible . 

 smell of sulphur; others, lixiviated, yield a mu- 

 riate of soda or ammoniac, or the sulphate of 

 iron; and often two or even the whole of these 

 salts are produced from the same sand. The 



