1845.] IMPERFECT FLUIDITY OF LAVAS EXEMPLIFIED. 85 



from the orifice of the crater, its fluidity rapidly diminishes, and 

 as it becomes more and more burdened by the consolidated slag 

 through which it has to force its way, its velocity of motion 

 diminishes in an almost inconceivable degree, and at length, 

 when it ceases to present the slightest external trace of fluidity, 

 its movement can only be ascertained by careful and repeated 

 observations, just as in the case of a glacier. In November 

 1843, I watched lava issuing rapidly from a small mouth in the 

 crater of Vesuvius at the rate of about one foot in a second. The 

 eruption of Etna in 1832 advanced at the rate of five miles in 

 two days, which is at the rate of one foot in about six seconds.* 

 We may contrast with this the eruption of Etna in 1614, which 

 yielded a lava which advanced but two miles in ten years, 

 according to Dolomieu,t during the whole of which time its 

 motion was sensible. This gives a mean rate of rather more 

 than three feet per day ; but at the conclusion it was no doubt 

 much slower. 



Mr. Scrope J saw the lava of 1819 in the Val del Bove 

 moving down a considerable slope at the rate of a yard a day, 

 nine months after its eruption. It had, he adds, the appear- 

 ance of a huge heap of rough cinders ; its progression was 

 marked by a crackling noise due to friction and straining, and, 

 " on the whole, was fitted to produce any other idea than that of 

 fluidity. In fact," he continues, " we must represent to our- 

 selves the mode in which the crystalline particles of lava move 

 amongst one another, rather as a sliding or slipping of their 

 plane surfaces over each other, facilitated by the intervention 

 of the elastic (?) fluid, than as the rotatory movement which 

 actuates the molecules of most other liquids." It is generally 

 conformable to this view that we find in Hamilton's Campi 

 Phlegrcei (fol. i. 38, Note] the curious remark that some lava is 



* E. de Beaumont, Recherches sur le Mont Etna. 



f Quoted by E. de Beaumont, p. 85. The original is in the Journal de Phy- 

 sique, vol. i. of the New Series, where it is mentioned that the same slowness of 

 motion has been observed in lavas of Vesuvius. Ferrara (Descrizione del Etna, 

 Palermo, 1818) denies this statement, but not I think on sufficient grounds. 



\ On Volcanoes, p. 102. 



