6o _ Experiments en Whinflone dnd Lava, 



took from the running ftream, being placed in the temperature of 22, loft its vitreous 

 character in two minutes, as already ilated ; and had the niafs itfelf been allowed to 

 remain but a very little longer in the ftream, it would certainly have acquired, as well as 

 the reft of the furface, the dull charadter of fcoria. 



The fame property accounts for the cruft which is formed on the furface of flowing 

 lavas, and which conftitutes fo remarkable a feature in their hiftory. Were lava to con- 

 geal after the manner of pitch or wax, by an uniform and gradual increafe of vifcidity 

 throughout, no cruft would be formed, or if, by the action of cold air, the upper 

 furface were to harden a little, it might he foftened again by an influx of frefti matter 

 a very little hotter than itfelf. In layas, however, as we have proved, when the furface 

 cools down to 21, it rapidly congeals to a hard fubftance, capable of refifting any heat 

 under 30. The cruft thus formed ferves as a pipe, within which the flowing lava is 

 confined. In feveral places on JEinz. we meet with vaft galleries, along which, 

 and out of which, the lava has flowed, leaving the cruft entire *. 



The irregular manner in which a lava flows, when not extremely heated, may likewife. 

 be referred to the fame caufe. On the lower part of the running ftream a cruft is formed, 

 fo ftrong as to retard its progrefs during a certain time, but the liquid behind, ac- 

 cumulating by degrees, at laft acquires fufficient ftrength to force open the cruft ; the 

 lava then flows out with rapidity, and continues its courfe till it is again retarded by the 

 formation of a new cruft. 



Thefe experiments feem to eftablifti, in a dire£l manner, what I had deduced, ana- 

 logically, from the properties of whinftone, namely, tha< the ftony chara£ter of a lava is 

 fully accounted for by flow cooling after the moft perfecl fufion ; and, confequently, that 

 no argument againft the intenfity of volcanic fire can be founded upon that charadler. 

 We are therefore juftified in believing, as numberlefs fails indicate, that volcanic. 

 heat has often been of excefllve intenfity. 



In the comparifon inftituted between whin and lava, the two clafles are found to 

 agree fo exadlly in all their properties which we have examined, as to lead to a belief of 

 their abfolute idenity. This idenity has been fully eftabliftied by Dr. Kennedy, who haa 

 performed an exaft analyfist of feveral of the very fpecimens of whinftone and lava men- 

 tioned in this paper; by which he difcovered, that the elements of the two clafles are the 

 fame : above all, that they both contain 4 or 5 per cent, of foda. Their agreement in this 

 eflTential circimiftance feems to account for their common properties, whilft the varieties of 

 proportion, among their component elements correfpond to the flight difitrences of 

 refult we have obferved between the individuals of the fame clafs %• 



So 



• As at Malpertui above Piedimoiite. 



f An account of Dr. Kennedy's analyfis is publiftied in this volume. (Edin. Tranf.) 

 J Though chemifts have hitherto overlooked, in their experiments, the mode in which bodies wrere cooled 

 after being reduced to a ftate of fufion ; yet many refults, which we are now entitled to afcribe to flow 

 cooling, have been occafionally obferved. The flag of a furnace bears a ftrong refemblance to what we 



have 



