SQ Prof. Bischof on the Natural History of 



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.* Davy found that a copper-wire of 

 2^5 of an inch in diameter, and a silver-wire of ^^ of an inch, 

 thrust into the lava near its source, instantly melted.-|- A wire 

 of copper J 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 doubtless heated far above its fusing point. 

 Now according to Daniel,^ silver melts at 2253° F., but copper 

 at 2548° F. ; we may therefore take a mean of 2282° F. 

 (=1000° R.) for the melting point of lava. 



Now, if we suppose the increase of temperature to con- 

 tinue to follow the same progression as has been discovered in 

 accessible depths, the lava must be in a state of fusion, accord- 

 ing to the observations near Geneva and in Cornwall, at the 

 depth of about 113505 feet, and from those in the Erzgebirge 

 at about 126829 feet below the level of the sea near Vesuvkis 

 or Etna.§ 



If we suppose steam to be the power by which the lavas are 

 raised from this enormous depth, and by which the volcanic 

 bombs, rapilli, and ashes are thrown up, and according to all 

 observations hitherto made, water in its elastic state seems to 

 be the only means by which the lavas jl and other volcanic 



* Thompson : Notices of an English Traveller, &c. Breislak. (Voyage dans 

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

 lava, the zinc melted ont, leaving the copper behind. 



+ Annal. de Chim. et de Phys. vol. xxxviii. p. 138. 



X Journ. of Science, xxiii. 



§ According to my observations made on a cooling basalt-ball of twenty-se- 

 ven inches diameter, and which I shall communicate afterwards, the increase 

 of temperature 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 progression being but very little greater than 1, this progres- 

 sion comes very near to an arithmetical one. The depths, above calculated^ 

 being but insignificant in proportion to the diameter of the globe, no great 

 error has been committed in supposing the increase of temperature follows 

 an arithmetical progression as far as these depths. With this exception, we 

 can hardly hope ever to become acquainted with the true progression of the 

 increase of the temperature to the interior. Therefore all such calculations? 

 as the former, can but give approximations to tlie truth. 



11 Von Humboldt's Reise, t. i, p. 186. A short time before the great erup- 

 tion of VesutiuSf in the year 1805, he and Gay-Lussac observed that the wa- 



