134 Trachyte and Basalt. paet i. 



_ ^ ■ 



does not seem to have been aware of any positive facts, 

 such as those above given ; and he has overlooked one 

 very necessary element, as it appears to me, in the 

 phenomenon — namely, the existence of either the 

 lighter or heavier mineral in globules or in crystals. 

 In a substance of imperfect fluidity, like molten rock, 

 it is hardly credible, that the separate, infinitely small 

 atoms, whether of feldspar, augite, or of any other 

 mineral, would have power from their slightly different 

 gravities to overcome the friction caused by their 

 movement ; but if the atoms of any one of these 

 minerals became, whilst the others remained fluid, 

 united into crystals or granules, it is easy to perceive that 

 from the lessened friction, their sinking or floating power 

 would be greatly increased. On the other hand, if all 

 the minerals became granulated at the same time, it is 

 scarcely possible, from their mutual resistance, that any 

 separation could take place. A valuable, practical dis- 

 covery, illustrating the effect of the granulation of one 

 element in a fluid mass, in aiding its separation, has 

 lately been made : when lead containing a small pro- 

 portion of silver, is constantly stirred whilst cooling, it 

 becomes granulated, and the grains or imperfect crystals 

 of nearly pure lead sink to the bottom, leaving a 

 residue of melted metal much richer in silver ; whereas 

 if the mixture be left undisturbed, although kept fluid 

 for a length of time, the two metals show no signs of 

 separating. 1 The sole use of the stirring seems to be, 



1 A full and interesting account of this discovery, by Mr. 

 Pp.ttinson, was read before the British Association in September, 

 1838. In some alloys, according to Turner (' Chemistry,' p. 210), 

 the heaviest metal sinks, and it appears that this takes place whilst 

 both metals are fluid. Where there is a considerable difference in 

 gravity, as between iron and the slag formed during the fusion of 

 the ore, we need not be surprised at the atoms separating, without 

 either substance being granulated. 



