ON THE SPHEROIDAL STATE. 



BY 



RICHARD J. MOSS, F.C.S. 

 (Read December 17, 1877.) 



In a paper on "The Penetration of Heat across Layers of Gas" 

 read at a recent meeting of the Society, Mr. Stoney* includes 

 Leidenfrost's phenomenon, or as M. Boutignyt calls it, the spher- 

 oidal state of volatile liquids, as an instance of the action of the 

 peculiar form of pressure which is exerted between hot and 

 cold surfaces when they are within a certain distance from one 

 another, depending on the length of the free path of the mole* 

 cules of the gas enclosed between the hot and the cold surfaces, 

 and on the difference of temperature of these latter. This 

 explanation of the spheroidal state places the phenomenon in an 

 entirely new light. It has hitherto been supposed that a volatile 

 liquid in this condition was sustained, and prevented from 

 touching the adjacent hot body by a rapid disengagement of 

 vapour from its surface. The explanation that was generally 

 accepted is very clearly expressed by Dr. Tyndall in his " Heat 

 as a mode of Motion," p. 154, where, referring to a spheroid of 

 water in a hot metallic basin, he says : — " The drop rolls about on 

 its own vapour — that is to say, it is sustained by the recoil of the 

 molecular projectiles discharged from its under surface. I with- 

 draw the lamp, and allow the basin to cool, until it is no longer 

 able to produce vapour strong enough to support the drop. 

 The liquid then touches the metal ; the instant it does so violent 

 ebullition sets in." 



It has, however, been regarded as a remarkable circumstance 

 that a quantity of vapour sufficient to produce the effect could 

 be given off by the liquid under the circumstances, for, as M. 

 Boutigny has shown, the spheroidal drop is always at a tempera- 

 ture below its boiling point, a condition that is not favourable 



* Transactions of the Royal Dublin Society, vol. 1 (new series), p. 13. 

 f Proceedings of the"Royal Institution of Great Britain, vol. I, p. 179. 



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