352 Prof. Bufi" f /i the Experiment «/Leidenfiost. 



surface beneath, the production of these figures would be an 

 impossibility. 



The greatest drops are not obtained upon such surfaces as 

 radiate heat best, but on those which possess the highest capacity 

 of conduction ; surfaces of silver are therefore preferable to all 

 others for these experiments. 



The phenomenon is produced with volatile liquids only ; the 

 higher the boiling-point of the liquid, the more sti-ongly heated 

 must be the surface on which it rests, in order to hold it in the 

 spheroidal state. The experiment therefore succeeds better with 

 aether than with water when bad conductors are used as vessels, 

 such, for example, as porcelain and glass. The lowest tempe- 

 rature at which the heated surface is not wetted by the liquid 

 poured upon it, must always be higher than the boiling-point of 

 the liquid. For example, in a silver basin at 75° C. aether, at 

 137° C. alcohol, and at 144° C. water first assume the spheroidal 

 form. 



In making these observations, I will not assert that Leiden- 

 frost's drop cannot be separated from the heated surface which 

 supports it. I believe, on the contrary, that even drops of con- 

 siderable magnitude may be torn from the surface by the force 

 of the developed vapour. I only affirm that such insulation is 

 not the necessary condition for the production of the phseno- 

 menon. This is caused, in my opinion, by the alteration produced 

 by heat in the relative attractions exerted by the surface of the 

 vessel upon the liquid, and by the liquid particles upon each other. 



The surface of a vessel will, it is known, be wet by a liquid 

 poured into it when the attraction of the matter of the vessel for 

 the liquid is greater than that of the particles of the liquid for 

 each other. In the reverse case the liquid asserts the spheroidal 

 form. The mutual attraction of the particles of the body is with- 

 out doubt diminished at a high temperature. If a silver basin be 

 heated with the water within it, the surface continues wet even 

 when the liquid boils. In fact, in this case the temperature of 

 the silver basin is only a little higher than that of the water. 

 The mutual attraction of the silver and the particles of water has 

 therefore been diminished almost in the same proportion as the 

 attraction of the water particles for each other ; the relation of 

 the attractions remains constant. If the silver basin be heated 

 beyond the temperature of boiling water, its adhesion for water 

 must diminish in a greater degree than the cohesion of the par- 

 ticles of the licjuid which still remains at its boiling-point. Hence 

 sooner or later a temperature will be attained, at which the co- 

 hesion overpowers the adhesion. The wetting of the surface 

 then ceases, and the spheroidal condition sets in, quite in har- 

 mony with the ordinary laws of capillarity. 



