Retarded Ebidlition — Boiler Explosions. 31 



selves, taking more or less time about it. Now, if it is only the 

 contact with the gaseous globules that excites ebullition, this 

 phenomenon ought to be seen even after the current ceases, 

 and the wires ought not to become inactive till the current 

 has ceased for some time. Experiments made in various ways 

 show that this is the case." 



When solids promote ebullition, Professor Dufour now con- 

 siders it may be owing to the gaseous matter that adheres to 

 them, and he thinks the power charcoal possesses of con- 

 densing gases on its surface, is the cause of its efficiency in 

 avoiding commotions during the ebullition of many liquids. 



' ' The phenomenon of ebullition," he states, " includes two 

 actions : a disengagement of vapour throughout the mass of a 

 liquid, which becomes passible as soon as the elastic force of 

 its vapour is equal to the incumbent pressure ; and the mole- 

 cular action, obscure in its intimate character, by which the 

 liquid is changed for the gaseous form." Contact with cer- 

 tain foreign bodies, and especially with gases, appears to give 

 rise to this molecular action, and consequently to determine 

 ebullition as soon as the state of pressure renders it possible." 



Professor Dufour considers that this molecular action is 

 distinct from cohesion, a force which opposes the division of a 

 liquid mass into smaller masses of the same kind. Steam is 

 not, he says, an aqueous dust, and in order to produce it some- 

 thing more than simple molecular separation is required. 



Passing more immediately to boiler explosions, Professor 

 Dufour observes that in a great many cases they occur when 

 an engine is at rest, and when the boiler has experienced a 

 certain amount of cooling. When this cooling occurs, it takes 

 place most rapidly above the surface of the water, and has the 

 effect of lessening the pressure of vapour that fills the upper 

 part of the boiler, and then, although perhaps seldom, the 

 ebullition of the water may be retarded as already explained, 

 and be made to begin suddenly by any concussion. 



Small quantities of sulphuric acid materially help the 

 retardation of the boiling point, and two explosions at Aber- 

 dare seem traceable to the employment of slightly acidulated 

 water. 



Professor Dufour considers that an apparatus to produce 

 an evolution of gas by electrolysis, or an arrangement for the 

 agitation of the water in boilers, might assist in preventing 

 their explosion. 



