Obsidianites. 447 



The physical characteristics of* Obsidianites accord well with 

 the hypothesis of their meteoric production. 



The average velocity of meteors whicli enter the atmosphere 

 may be taken as about 40 miles per second. If only one per cent, 

 of the energy which such meteors possess were, under the influ- 

 ence of air-friction, converted into heat and retained by the 

 body, it would probably be sufficient to raise the substance of an 

 obsidianite to the melting point and render it completely liquid. 

 The melting point of the material and its specific heat have been 

 determined in the Physical Laboratory, Melbourne University, 

 as 1324 deg. C. and .21 respectively. The rem.irkably homogen- 

 eous quality of the glass of which obsidianites are composed 

 renders it certain that they have, prior to assuming their 

 present form, been fused throughout. 



The forms which a mass of liquid motion is capable of assum- 

 ing have been the subject of discussion by many eminent mathe- 

 maticians from the time of Newton to the present day. Neg- 

 lecting tliH ellect of ;iir-resiscance on the surface it lias l)een 

 shown that the following forms are possible : — 



(1) The sphere — possible only when there is no rotation. 



(2) The oblate spheroid — stable at low speeds of rotation. 



(3) The prolate spheroid — stable, if at all, only at high 



speeds of rotation. 



(4) The apioid, or pear-shaped figure of revolution. 



(5) The dumb-bell or hour-glass figure of revolution. 



It is remarkable that all these forms, if we ignore secondary 

 features, are comprised among those assumed by Obsidianites. 

 The occurrence of cigar-shaped and dumb-bell shaped figures is 

 of particular interest since the stability of these types is still 

 a matter of dispute among mathematicians. 



Tlie secondary features of form alluded to consist of the well- 

 known rim. the ripples concentric with it on one face and the 

 smaller pittings and furrowings of the surface. These features 

 have been satisfactorily explained as due to the action of the air 

 on the moving liquid, and Suess has succeeded in obtaining 

 artilicial pittings, etc.. by the action i»f jets of steam on rotating 

 buups of resin. 



I have also observed on two specimens umrkings which have 

 strongly the appearance of having been produced by the impact 

 of the glass, while still plastic, on some hard object. 



