228 



but we must distinguish, with Spallanzani, be- 

 tween the pumices which draw their origin direct- 

 ly from primitive rocks, and those which, being- 

 only altered volcanic productions, vary like them 

 in their composition # . A certain state, into which 

 several heterogeneous substances pass, or the 

 result of a particular mode of action, are insuf- 

 ficient to establish a species in the classification 

 of simple minerals. 



The experiments of Mr. Da Camara, and those 

 I made in 1802, come in support of the opinion, 

 that the pumice stones adherent to the obsidians 

 of the Peak of Teueriffe do not unite to them 

 accidentally, but are produced by the expansion 

 of an elastic fluid, which is disengaged from the 

 compact vitreous matter. This idea had for a 

 long time occupied the mind of a person highly 

 distinguished for his talents and reputation at 

 Quito, who, unacquainted with the labors of the 

 mineralogists of Europe, had devoted himself to 

 researches on the volcanoes of his country. Don 

 Juan de Larea, one of those lately sacrificed 

 to the fury of faction, had been struck with the 



Voy. aux lies Ponces, p. 122 ; Voy. aux Iks de Lipari, p. 

 88. 



* The word lam is still more vague than that of pumice 

 stone. " It is as little philosophical to require an exterior 

 description of lava, as a mineral species, as it is to ask the 

 general characters of the mass, that fills the veins of ore.'* 

 Leop. Von Bucb, Geognost. Beob. vol. ii, p. 17a. 



