Considerations on Volcanoes. 359 



solidates and cracks, emitting vapours, and thus resembling the 

 scoria of furnaces. He considers that the low degree of this heat 

 is proved by the slight effect which a current of lava produces on 

 a thermometer held near it ; and Dolomieu's opinion is adduced 

 to confirm the supposition, that the mobility of the particles of lava 

 arise from some other cause than their complete combination with 

 caloric. 



The author considers this opinion to be further proved by ex- 

 periments on lava, in furnaces. Here, it becomes a glass ; whereas 

 in its natural state of flowing and consolidation, it is an aggregate 

 of crystals or separate substances, and thus possesses a lithoidal 

 character. Hence also, he concludes that the crystals found in it 

 existed previously as solid substances in this apparent fluid, and 

 have not been formed during the process of cooling. His con- 

 clusion, to quote his own words, is, " that this substance is seldom 

 in a state of igneous fusion, but owes its partial and imperfect 

 Hquidity to some other cause, by which a certain mobility is com- 

 municated to the solid crystalline particles of which it consists." 



He considers, therefore, that its liquidity will depend, not 

 upon the quantity of caloric with which it is combined, but on the 

 quantity of fluid which it contains in proportion to the size and 

 form of the solid particles ; and that its condensation is effected, 

 not by parting with its heat, but by the escape or condensation of 

 the fluid which caused its mobility. And as it discharges much 

 elastic vapour on cooling, he considers this matter to be the cause 

 of its fluidity ; while the fissures then formed, allowing more 

 elastic vapour to escape from the interior parts, the same process 

 goes on through the whole mass. 



The elastic vapour in question he considers to be also that 

 which is the cause of all the violence and explosion in the volcano 

 itself, or steam ; whatever other substances in less quantity may 

 be united to it. Bitumen, as it appears by Mr. Knox's experi- 

 ments, is one of those substances. The mobility and ebullition 

 of lava are therefore the produce of water in union, disengaged 

 in the form of vapour; and thus, when this is disengaged, 

 the solid parts, held in mixture rather than fluidity, unite and 

 become rock. And, consequently, as the volume of vapour gene- 

 rated on any point will depend on the ratio of the temperature 

 and pressure, so, under the same circumstances in these respects, 

 the fluidity of the lava will vary according to the degree of com- 

 minution of its Component crystalline particles. 



The general conclusions flowing from these considerations are, 

 that the volume of vapour generated by any change in the cir- 

 cumstances of a lava, depends on the ratio of the temperature to 

 the force of compression — that the intumescence or dilatation of 

 Java, varying with the volume of vapour produced, must follow 

 the same law — that the proportion of the vapour generated, which 



