30 Retarded Ebullition — Boiler Explosions. 



room, or tlie shaking occasioned by walking over the floor." 

 Professor Dufour compares this action to the effect of agita- 

 tion in exciting crystallization in super saturated solutions of 

 sulphate of soda. 



After reference to experiments of M. Marcet and others, 

 the Professor observes that if the presence of a layer of 

 gaseous matter on the surface of a liquid excites its ebullition, 

 it must be interesting to know what effect would follow from 

 keeping a gaseous layer constantly renewed on the surface of 

 a body plunged into water. To ascertain this he immersed 

 two platina wires in water, and rendered them inactive by 

 repeatedly boiling the liquid. It was then possible to obtain 

 retardation of the boiling point to the extent of 10° or 15° C. 

 A galvanic current was then transmitted through the wires, 

 and gases were continually evolved from their surfaces, and 

 it became impossible to obtain the least retardation of the 

 boiling point. The currents of oxygen and hydrogen starting 

 from the two electrodes, acted as provocatives to ebullition, 

 the moment the change of pressure rendered that pheno- 

 menon possible.-" When ebullition had been retarded to the 

 extent of 15° or 20°, and was suddenly excited by the electric 

 production of these gaseous currents, an instantaneous commo- 

 tion occurred as if gunpowder had been ignited. 



It might be thought the electric current and not the evolu- 

 tion of the gases occasioned these effects ; but Professor Du- 

 four thinks the following experiments show that the gas is 

 the effective agent in the case. " When a certain quantity 

 of water covered with oil is heated, by means of a bath, in a 

 vessel of porcelain which has for some time previous held 

 sulphuric acid, it is easy to obtain under a normal pressure a 

 retardation of 6° or 7° 0. If thin platina wires, that have 

 been rendered inactive by prolonged contact, are introduced, 

 the facts just cited may be observed, namely, retardation 

 of ebullition so longl as the current does not pass, and its 

 excitation as soon as the current does pass. The ebullition 

 thus produced commences immediately round the platina 

 wires only, and not in the space between them that forms part 

 of the circuit. If the platina wires are replaced by copper 

 wires, also made inactive, ebullition commences about the 

 negative pole only, where hydrogen is evolved ; the positive 

 pole where the oxygen unites itself to the copper, and where, 

 consequently, no gas escapes, remains perfectly quiet, and is 

 not the seat of any ebullition.'" 



" When the electrolysis is continued for a few minutes, 

 and the surface of the platina wires become covered with ad- 

 hering globules of gas, which remain even after the interrup- 

 tion of the current, these globules gradually detach them- 



