Prof. J. W. Judd — On Marekanite and its Allies. 243 



transparent Eupert's drops between crossed Nicols, the Marekanite 

 was found to be precisely similar in its behaviour. 



In the second place, ten nodules of Marekanite, varying in size and 

 colour, were selected for cutting by the lapidaries' wheel ; and their 

 behaviour, carefully watched, was as follows: — 



One, the largest, was cut through without undergoing fracture. 



Six, while being cut, had one or more cracks developed in them, 

 and the formation of these cracks was accompanied by distinctly 

 audible, snapping sounds. 



In the remaining three cases, snapping sounds were heard from 

 time to time while the cutting was taking place, and when the mass 

 was nearly cut through, a distinct explosion occurred, the fragments 

 being thrown to the distance of several yards. 



In the case of two Rupert's drops, treated in precisely the same 

 manner, it was found that before the saw had passed half through 

 them, an explosion occurred, which was more violent than that accom- 

 panying the breaking up of the Marekanite-nodules. These facts 

 point to the conclusion that the glass of the Marekanite-balls is in a 

 condition of intense strain, in some cases approaching that which 

 characterizes the Eupert's drops. 



In the case of both the Eupert's drops and the Marekanite-balls, 

 the portion left embedded in the cement after the explosion was 

 found, to be traversed by a system of radial cracks, and on the 

 attempt being made to grind them they fell to pieces. The fine 

 splinters into which the Marekanite-balls broke up were found, 

 when examined by the microscope, to be quite similar to those 

 derived from the Eupert's drops. Acting upon a suggestion made 

 to me by my colleague, Prof. F. G-uthrie, F.E.S., I annealed some 

 of the Marekanite-balls by heating and slowly cooling them. They 

 were then found to have lost their power of depolarizing light, and 

 were easily cut and ground into thin sections. 



I must now proceed to describe another set of remarkable 

 phenomena — those namely which are exhibited by the Marekanite- 

 nodules when they are heated. 



If one of the balls be subjected to a gas-jet, blown by a moderately 

 strong air-current, it will be seen that, as redness is approached, 

 thin films of glass detach themselves from the surface of the ball 

 and these films are successively broken up into fine particles, so that 

 there is maintained a constant dispersal of excessively minute glassy 

 dust, which is thrown to a considerable distance. If the temperature 

 be now raised to whiteness, the whole mass swells up in cauliflower- 

 like excrescences, till it has attained eight or ten times its original 

 bulk. The resulting white mass is found to be a true pumice, which 

 floats upon water, and microscopic sections of it are indeed quite un- 

 distinguishable in appearance from many natural pumices. Some 

 of the Marekanite-balls, however, exhibit this property in a much 

 more striking manner than do others. 



These experiments prove that the quantity of volatile ingredients 

 in these Marekanite-balls is very great, sufficient indeed, on being- 

 liberated by heat, • to convert the obsidian into a pumice. Herter 



