Prof. Buff on the Experiment of Leidenfrost. 353 



Perhaps, my dear Sir, you will be inclined to agree with, this 

 manner of viewing the subject ; if so, I entertain the hope that 

 you will excuse this somewhat elaborate statement of the question. 

 With great esteem, I have the pleasure to subscribe myself. 



Yours most sincerely, 

 Giessen, Sept. 29, 1855. IJ. BuFF, 



With reference to the subject of Prof. Buffos interesting 

 letter, I may be permitted to say that I have never published 

 anything on the experiment of Leidenfrost, though I have taken 

 some pains to instruct myself as to the cause of this sin- 

 gular phsenomenon. At an early period of railway travelling, 

 Mr. George Stephenson conceived the idea of making his loco- 

 motives rest on steam springs. The steam within the boiler 

 formed an elastic cushion for the support of the boiler. My 

 beUef is, that the spring suggested by the great engineer is pre- 

 cisely that which supports the drop in the experiment of Leiden- 

 frost; the liquid, 1 believe, rests upon a cushion of its own 

 vapour. Some of my reasons for agreeing with those who hold 

 this belief are as follows : — 1. The incessant motion of the drop 

 necessitates a continuous expenditure of force, and this force is 

 only attainable by the production of vapour underneath the drop, 

 which, as is well known, speedily diminishes in bulk in conse- 

 quence of the evaporation. 2. In a deep concave vessel, the 

 vapour escapes laterally from the liquid as it encounters the 

 least resistance in this direction ; but if the liquid be suffered to 

 spread itself over a comparatively flat surface, or better still, if 

 the surface be rendered a little convex towards the centre of the 

 drop, the vapour finds the resistance in a lateral direction greater 

 than that in a vertical direction, and the steam actually breaks 

 through the thin liquid layer above it and escapes at the centre. 

 The existence of the vapour beneath the drop is thus easily 

 reduced to demonstration. 3. If an interval really exists between 

 the drop and the surface, it occurred to me that if this interval 

 could be made manifest to the eye it would tend to settle the 

 question. With this view I made the following experiment : — A 

 silver basin was inverted, and the polished convex surface slightly 

 dinted so that a drop might rest upon it. A fine platinum wire 

 was stretched vertically behind the basin and united with the 

 poles of a small voltaic battery ; the wire was in this way ren- 

 dered vividly luminous. A drop of deeply black ink was placed 

 upon the surface of the basin ; bringing the eye on a level with 

 the bottom of the drop, and looking towards the platinum wire, 

 the latter was seen through the .space between the drop and the 

 bottom of the basin*. In this way the under surface of a drop, 



* A little spirit mixed with the ink renders the experiment more easy. 



