1869.] Vesuvius. 195 



The smaller fragments shot up vertically into tlie air are re- 

 duced by mutual friction to a sort of gravel called " lapillo ; " still 

 further comminuted into sand " puzzolana," and when reduced to 

 fine dust, ceneri, or " ashes." 



The distance to which these finely-divided ashes have been 

 carried is prodigious. During one of the early eruptions of Vesu- 

 vius, A.D. 472, its ashes were carried as far as Constantinople, 

 where the singular event was afterwards commemorated annually 

 on the eighth of the Ides of November. 



Among the sublimated mineral products of Ycsuvius and its 

 neighbourhood we must not omit to mention Sulphur ; it enters 

 most largely into the composition of volcanic mineral products, and 

 also occurs in a pure state deposited in joints and fissures, esjiecially 

 round the remarkable crater called the Solfatara, near Pozzuoli, 

 one of the all-but-extinct volcanic vents of the " Campi Phlegraei," 

 west of Naples. 



Yast beds of this mineral (deposited from the vapours of sul- 

 phuretted hydrogen) occur both in Iceland and Sicily, and are of 

 great commercial value. 



We come then to the active agent in all volcanic outbursts — 

 "Water. " Water," says Mr. Scrope, " we know is converted into 

 vapour only at temperatures increased in proportion to the in- 

 creased pressure to which it may be subjected ; and when altogether 

 hindered from communication with the atmosjDhere, as in a Papin's 

 digester, or other closed vessel, may be made red hot without ex- 

 panding into vapour.* The moment, however, that an opening is 

 made in the enclosing vessel, reducing the j)ressure to that of the 

 atmosphere only, it flashes instantly into steam with exj)losive 

 violence. The same efi'ect of course must take place in an im- 

 perfect liquid or paste composed of water and any sohd matter in 

 mechanical suspension or mixture, such as flour, clay, sand, or any 

 other granular substance." 



* The Annalcs de Chimic, II., xxi. and xxii., record the experiments of Cagniard 

 de Latoiir on tlie expansion of liquids under pressure. 



Do Latour partially filled some strong glass tubes with water, with alcohol, 

 with ether, &c., and furnished them witii gauges, and hermetically sealed thorn. 

 He then cautiously raised the temperature. The alcohol (sp. gr. 0'844), which 

 occupied two-fifths the capacity of the tube, gradually expanded to double its 

 volume, and then suddenly disappeared in vapour, at a temperature of 497° Fahr. : 

 it then exerted a pressure of about 119 atmospheres. 



Water was found to become gaseous in a space equal to about four times its 

 original bulk, at a temperature of about 773° (that of melting zinc). So great was 

 the solvent power of water on glass, at this high temperature, that the addition of 

 a little carbonate of soda was necessary to diminish the action on the glass, whicli 

 frequently gave way until this expedient was adopted. As the vapours cooled, a 

 point was observed at which a sort of cloud filled the tube, and in a few moments 

 after the liquid suddenly reappeared. 



From these experiments it would appear that there is a point in the tem- 

 perature of every liquid at whicli no amount of pressure is sufficient to retain it in 

 the liquid form. 



