Evaporation and Steam-Boiler Explosions. 31 



tion of evaporation in all its forms. From this we see that 

 a fluid mass, without interior or exterior surfaces, or so in- 

 closed as virtually to answer this description, might be heated 

 up far above the boiling-point without boiling. "We see that 

 ebullition is but the effect of an internal evaporation starting 

 in minute air-bubbles, and growing with the expanding bub- 

 ble. We see that water entirely freed from air-bubbles, and 

 with a restricted open surface, as in Donny's tube experi- 

 ment, should go on heating up far above the boiling point, 

 until at last the whole heated mass would flash into steam 

 with an explosion. All the phenomena described by Donny, 

 in his excellent paper in the Annates de CTiimie et de Phy- 

 sique, follow as easy and obvious deductions from this con- 

 stitution of the fluid surface. Indeed, we do not at all wonder 

 his being forced, from his experiments, to conclude empiri- 

 cally that there must be some peculiar quality in surfaces, 

 which makes evaporation take place so much more readily 

 on them than in fluid masses. We see, too, how utterly 

 fallacious are the experiments usually taken, as measuring 

 fluid cohesion — .they being in fact only results of the weak 

 cohesion in surface layers — which, with the free mobility of 

 fluid parts, fully explains all the observed results. This fully 

 explains how a too perfect boiling of the mercury in barome- 

 ter tubes makes it adhere at the top with such tenacity. It 

 explains Berthollet's experiment on the forced dilatation of 

 fluids, in which a deaerated fluid, sealed when hot, does not 

 shrink in cooling for a long time, but at last breaks and col- 

 lapses — indicating that it has borne a great tension before 

 yielding. Prof. Henry's elegant experiments with soap-bubbles, 

 in which by measuring the tension of the inclosed air, he is able 

 to deduce, first, the compressing force, and thence the cohe- 

 sion of the fluid film, with a very great value, furnish an in- 

 dependent confirmation of the same general views. We may 

 remark that the heterogeneous structure of the outer layers 

 would destroy the mobility of their parts, and give a film- 

 like character to the fluid surface, while all within this film 

 would have free mobility. This, with the additional fact of 

 a drawing inward of the outer layers, by the unbalanced co- 

 hesive action of the layers near the surface, explains the 



