196 Dr. M'Keever on [March, 



Florentine academicians were enabled to burst a brass globe, the 

 cavity of which was an inch in diameter, by means of this expan- 

 sive power, and calculating from the tenacity of brass, and the 

 thickness of the sides of the globe, this must have required a 

 force exceeding 27,720 pounds.* Vast masses of rock are from 

 this cause frequently detached from the cliffs, and precipitated 

 with great velocity into the adjoining valleys. The rending of 

 trees in northern latitudes during the winter months, the bursting 

 of water pipes, the raising of pavements, the increased fertility 

 of the soil after frost (owing to the facility with which the deli- 

 cate fibres of the roots of plants are enabled to extend them- 

 selves), may all be referred to the same principle. The expan- 

 sion which water undergoes at elevated temperatures is, however, 

 still more considerable, and is capable of producing the most 

 astonishing effects. When heated to 212° of Fahrenheit, at 

 the mean pressure of the atmosphere, it becomes converted into 

 steam, an elastic substance, the bulk of which, even at that tem- 

 perature, is about 1800 times that of the water from which it 

 originated. When heated beyond this point, vapour is expansi- 

 ble to a wonderful degree. Count Rumford ascertained by 

 incontrovertible experiments, that its elasticity became doubled 

 by every addition of temperature, equal to 30° of Fahrenheit's 

 scale. With the heat of 212 + 30 = 242, its elasticity he found 

 equal to the pressure of two atmospheres; at the temperature of 

 242 + 30 = 272, it will be equal to four atmospheres, and so 

 on.f When suddenly generated, this powerful agent is capable 

 of producing the most violent, and at times the most destructive 

 effects. Thus instances J have occurred, where, in consequence 

 of a person carelessly spitting into a copper foundry, the entire 

 building has been destroyed, and even when the moulds contain 

 the slightest moisture, the melted metal is driven back with a 

 loud report, and is violently dispersed in every direction. A 

 drop of water also by falling into a vessel of boiling linseed oil, 

 where it became instantly converted into vapour, has been known 

 to produce the most violent explosions. On this principle then, 

 namely, that of sudden expansion, I conceive the whole process 

 admits of explanation. The minute, but numerously interspersed 

 globules of water, which had for ages been percolating through 

 the pores and fissures of the rock, undergoing, by the application 

 of a large fire to its base, a sudden enlargement of volume in a 

 short time acquired such a degree of expansive energy as to burst 

 asunder the strong confines by which they had so long been 

 surrounded. Now were Hannibal, while the fire was imparting 



* See Murray's Chemistry, vol. i. p. 250. 



-}- The elastic force of steam does not increase in quite so rapid a progression as Count 

 Rumford affirms ; according to the late experiments of Dr. Ure, it is equal only to 53' 6 

 inches of mercury at 242°, and to about 89 only at 272°. — (See Philosophical Trans- 

 actions, 1818, or Annals, vol. xiii, p. 215.) — Edit. 



$ See Black's Lectures, vol. i. 



