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phenomena exhibited by obsidians exposed to a 

 white heat. He had thought, that, wherever 

 volcanoes act in the centre of a country covered 

 with porphyry with base of obsidian, the elastic 

 fluids must cause a swelling of the liquified mass, 

 and act an important ^part in the earthquakes 

 preceding eruptions. Without adopting an opi- 

 nion, which seems somewhat bold, I made, in 

 concert with Mr. Larea, a series of experiments 

 on the tumefaction of the volcanic vitreous sub- 

 stances at TenerifFe, and on those which are 

 found at Quinche, in the kingdom of Quito. To 

 judge of the augmentation of their bulk, we 

 measured pieces exposed to a forge fire of mo- 

 derate heat by the water they displaced from a 

 cylindric glass, enveloping the spongy mass with 

 a thin coating of wax. According to our expe- 

 riments, the obsidians swelled very unequally : 

 those of the Peak and the black varieties of Co- 

 topaxi and of Quinch6 increased near five times 

 their bulk. The swelling on the contrary was 

 very little perceptible in the obsidians of the 

 Andes, the color of which is a brown approach- 

 ing to red. When the reddish variety is mingled, 

 in thin plates, with the black and blackish gray 

 obsidians, the striated mass resembles porcelain 

 jasper * ; and the opake parts resist the action 

 of the fire for a length of time, while those which 



* Porzellan-jaspis of Werner; thermantide porcellanite ol 

 Haiiy. 



] 



