Retarded Ebullition — Boiler Explosions. 29 



times exceeding 20° and even 30° 0. After a third re-heating 

 it is seldom that boiling occurs at the normal point." 



Professor Dufour then tabulates his experiments, showing, 

 according to the circumstances just explained, retardation of 

 the boiling point ranging from 1 0, 7 0. to 23° - 2 0. He then 

 adverts to the well-known fact that the presence ofplatina wire 

 assists ebullition ; but he tells us that, " while it is rare, after 

 a first or even a second heating, that water containing platina 

 remains calm and liquid, when a diminution of pressure 

 affords conditions under which it might boil ; if it is exposed 

 to prolonged and successive re-heating, retardations at last 

 appear as in the preceding cases. The platina ceases to 

 excite a change of condition, and behaves at the bottom of the 

 water just like the walls of a glass vessel. We thus see what 

 chemists have often noticed, that platina, having served for 

 some time as a preventive against concussions, at last becomes 

 inactive and the pieces of wire have to be renewed." 



In former experiments Professor Dufour showed that 

 globules of water heated beyond 100° C. when surrounded by 

 other fluids of the same density, boiled furiously if touched 

 with pieces of wood, paper, cotton, etc., and he now finds that, 

 like platina, these various substances lose their power by fre- 

 quent or continuous use. Partial renewals of the water occa- 

 sioned diminution in the retardation of ebullition. 



Professor Dufour .remarks that, according to experiments 

 hitherto made on the retardation of the boiling point of water, 

 it has been supposed that this effect is only witnessed in 

 vessels of glass or porcelain. He adds in a note, that M. 

 Magnus records an instance of retardation in a vessel of pla- 

 tina, and goes on to say that when ebullition is excited by 

 diminishing pressure, water in contact with divers metals 

 retards its boiling point, and thus the mere contact of a me- 

 tallic surface is not sufficient to counteract its tendency to 

 maintain the liquid state. 



" When water is in a state of retarded ebullition it pre- 

 sents, in appearance at least, no special activity, although a very 

 abundant and exceptional surface evaporation is really going 

 on. It looks motionless and calm ; no bubbles of gas or vapour 

 disengage themselves from the mass or from the walls of the 

 vessel. This liquid condition is analogous to an instable 

 equilibrium, and ebullition may supervene all at once. The 

 sudden transformation of a portion of the liquid into vapour 

 sometimes occurs without any appreciable external cause ; but 

 we are nearly sure of provoking it, by giving a shock to the 

 vessel, and sometimes we can do so by admitting a small 

 quantity of air. It is not rare to see ebullitions follow 

 a tolerably loud noise, such as a blow struck in an adjacent 



