240 PROCEEDINGS OF THE AMERICAN ACADEMY 



in any case. lie referred to the act of plunging the moistened 

 hand into masses of molten metal as coming under this head, 

 and as having been repeatedly performed in this country more 

 than twenty years ago. The explanation he gave of the 

 safety of the hand in this exposure was, that the moisture, 

 volatilizing, rendered a part of the heat latent, and encased the 

 hand in a sheath of aqueous vapor, so that only radiant heat 

 acted upon it, and that only for the instant the hand was im- 

 mersed. 



As proof of the occurrence of the spheroidal state in the 

 absence of temperature, Professor Horsford instanced the form 

 mercury assumes on glass, that of oil and ether on water, and 

 the bead upon acohol when agitated. The explosion at the 

 close of the experiment of burning potassium or sodium on 

 water, especially where the piece is large, he ascribed to the 

 same cause as the explosion in the Leidenfrost experiment, 

 where the cooling of the highly heated surface permits contact. 

 The explosion attending the contact of fused saltpetre and 

 water he classed with the Leidenfrost experiment. That 

 which sometimes takes place when potassium is thrown into 

 water or nitric acid, but an instant after the contact, he as- 

 cribed to another cause, — the mixture of hydrogen from the 

 decomposed water, and oxygen from the air or acid, in such 

 proportions as to be explosive. 



Professor Horsford expressed a doubt whether any explosions 

 of steam-boilers were to be ascribed to the Leidenfrost phenome- 

 non, alleging that the temperature of 300^, which is about the 

 temperature permitting contact, cannot produce sufficient steam 

 from within the boiler to effect an explosion. 



Professor Horsford concluded his paper with a series of ex- 

 periments illustrating the general subject. 



Professor Peirce stated that he had obtained from some in- 

 vestigations connected with the turbine wheel the following 

 result : — that the curve along which a material point should 

 move so as to compel this curve to raise Aveights to which it 

 is attached, must be the cycloid. He exhibited a drawing 



