252 Dr Charles Daubetiy on the 



Appendix II. (Page 236.) 

 Abich's ObservatioTis on the formation of Obsidian and Pumice. 



The mode in which pumice and obsidian have been formed, can 

 only be cleared up, through a more exact scrutiny into the nature of 

 the volatile materials which both these bodies, in greater or less 

 quantity, contain. Humboldt and others have remarked upon the 

 swelling out which certain obsidians undergo at a white heat, and 

 have found a great difference, in this respect, to exist between spe- 

 cimens taken from different localities. — Abich has further ascer- 

 tained, that the poorer in silica, and the richer in alkaline bases ob- 

 sidian is, the more, in short, its composition approaches to clinkstone, 

 the more readily it may be made to pass by heat into the condition 

 of pumice. 



It being admitted that the immediate cause of the change is the 

 extrication of some volatile principle, the nature of this latter be- 

 comes the next subject for inquiry. 



Abich found, that, in order that the mineral should swell out into 

 a porous mass, it must be submitted to heat in lumps ; for, if it be 

 in powder, it does not undergo any such tumefaction by exposure to 

 a high temperature, but only changes to a dark red or brown colour ; 

 losing, during the process, twice as much weight as the same had 

 done in lumps ; from which it would appear, that only a portion of 

 those volatile ingredients which escape from the powder is, in the for- 

 mer case, disengaged. 



By comparing the analysis of obsidians with that of pumices ob- 

 tained from the same locality, it would appear, that although the 

 sum of the alkaline bases is in both very nearly the same, yet that 

 there is more potass in obsidian, more soda in pumice, — the increase 

 in one alkali corresponding with the deficiency in the other. 



This might lead one to conjecture, that, during the formation of 

 pumice, a certain amount of potass was dissipated, and a proportion- 

 ate quantity of soda introduced from without, and that, from the dis- 

 engagement of the former, the cellular condition of the mineral might 

 have resulted. 



It will be readily supposed, that both obsidians and pumices are 

 often very widely different one from the other in their constitution, 

 and that amongst the latter the darker and more cellular varieties may 

 arise from a predominance of earthy and alkaline bases, the white 

 and silky-looking kind from a larger amount of silica. 



When felspathic rocks, rich in alkali, pass into a state of fusion in 

 the presence of some earthy base, the latter displaces a portion of the 

 alkali, and thus causes the mass to swell out. 



Jt is worthy of remark, that chlorine and water are present in all 

 pumices and obsidians, and that, from many, certain inflammable 

 gases are also disengaged. This latter fact, indeed, has led some 

 chemists to imagine, that the cellularity of pumice may have been 



