240 Natural History of Volcanos and Earthquakes. 



suvius of the year 1785, melted at 18° of Wedgewood's pyrome- 

 ter, lava from Torre del Greco not till 40°. But their fusibility 

 varied very considerably, according as the melted lava had been 

 cooled rapidly to a glass, or more slowly to a stony crystalline 

 mass. Thus, for example, those two lavas, when in the form of 

 a glass, both melted at the same degree, (18°,) whilst the lava of 

 1785 was less fusible than that of Torre del Greco, when of a 

 stony nature.* From other appearances it may, in general, be 

 concluded, that the fusibility of lavas is between that of silver 

 and copper. Thus in the lava which destroyed Torre del Greco, 

 some gold and a few copper coins were found unmelted ; but the 

 silver coins were melted and baked together with some copper 

 coins.f Davy found that a copper-wire of ^V of ^^ i^^^ch in diam- 

 eter, and a silver-wire of ■^\ of an inch, thrust into the lava near 

 its source, instantly melted.^ A wire of copper ^ of an inch in 

 diameter, which I held in a stream of fused basalt, flowing out 

 from a furnace, melted immediately. But the basalt was doubt- 

 less heated far above its fusing point. Now according to Daniel,<§» 

 silver melts at 2233° P., but copper at 2548° F. ; we may there- 

 fore take a mean of 2282° F. ( = 1000° R.) for the melting point 

 of lava. 



Now, if we suppose the increase of temperature to continue to 

 follow the same progression as has been discovered in accessible 

 depths, the lava must be in a state of fusion, according to the ob- 

 servations near Geneva and in Cornwall, at the depth of about 

 113505 feet, and from those in the Erzgehirge at about 126829 

 feet below the level of the sea near Vesuvius or Etna.\^ 



* Glass is well known to be acted upon in a similar manner. When converted, 

 by being melted and slowly cooled again, into Reaumar's porcelain, it becomes less 

 fusible. 



t Thompson : Notices of an English Traveller, «&c. Breislak. (Voyage dans la 

 Camp., vol. i, p. 279,) mentions, that when bell-metal was plunged into the lava, 

 the zinc melted out, leaving the copper behind. 



:j: Ann. de Chim. et de Phys. vol. xxxviii, p. 138. 



§ Journ. of Science, xxiii. 



II According to my observations made on a cooling basalt-ball of twenty-seven 

 inches diameter, and which I shall communicate afterwards, the increase of tem- 

 perature from the surface towards the centre of the earth, seems to take place, not 

 in an arithmetical, but in a geometrical progression. But the exponent of this pro- 

 gression being very little greater than 1, this progression comes very near to an 

 arithmetical one. The depths, above calculated, being but insignificant in propor- 

 tion to the diameter of the globe, no great error has been committed in supposing 



