﻿Intelligence and Miscellaneous Articles, 323 



heated considerably above its temporary boiling-point ; the conditions 

 are therefore favourable to the occurrence of a Leidenfrost's drop. 



The experiment was made, and succeeded with the greatest ease. 

 When the evacuation extended to a pressure of less than 10 centims., 

 and the water-bath was raised to 90° C. or more, spheroids were 

 obtained which moved briskly about. I was not concerned to as- 

 certain what was the magnitude of the greatest spheroids which 

 could thus be obtained ; approximately the lowest temperature at 

 which the drops maintained themselves was 83° C, while the mano- 

 meter oscillated about 2 or 3 centims. Of course the tension 

 increases when the evaporation commences upon the hot dish, and 

 must again be diminished by pumping, which can scarcely be effected 

 without the mercury oscillating. Sudden and violent formation of 

 vapour, which might have endangered the manometer, has not oc- 

 curred in my experiments, of which, to be sure, there were only three, 

 and the quantities of water but small. 



In repeating the experiment, it will always be advisable to keep 

 the dish so far horizontal that the drop in its movement cannot touch 

 the glass of the bell-jar. The bell-jar ultimately becomes dim from 

 deposition of moisture, but at the lower and therefore warmer part 

 remains long enough transparent to enable several drops to be suc- 

 cessively observed. I think this experiment is decisive for the expla- 

 nation of Leidenfrost's phenomenon ; for it shows that the force 

 which supports the drop obeys the laws of the pressure of vapours. 



I may be here permitted to bring before a wider scientific public 

 some observations on the shapes of Leidenfrost's drop which in March 

 1869 1 communicated to the Niederrheinische Gesellschaft fur Natur- 

 und Heilkunde. As regards the formation of the star-shapes I adhere 

 to Berger's explanation, with the addition, however, that the forma- 

 tion of the star is a phenomenon of vibration, and one accompanied 

 by a remarkable formation of nodes. The motion in question is 

 alternate extensions and contractions, usually occasioned by the col- 

 lection of vapour under the centre (which causes the drop to spread 

 out), brought into regular course and maintained by causes which 

 Berger has described. The simplest vibration is that in which the 

 drop extends and contracts as a whole ; it then represents alternately 

 a more flattened and an elongated spheroid. This modification is 

 most easily witnessed with very small quantities of liquid. 



If the drop has a larger area it behaves like any other vibrating 

 body, and divides into aliquot parts, forming nodes and loops. The 

 simplest division is that into quarters. It therein extends first in one 

 direction so that its section is almost an ellipse. In the following 

 period the larger axis contracts, and the former smaller axis extends, 

 so that the section forms an ellipse at right angles to the first. It 

 therefore oscillates between two crossed ellipses ; and when the motion 

 is rapid enough it is observed as a cross. When the amplitude of the 

 oscillation is sufficient, the contraction in the middle extends so far 

 that the section is biscuit-shaped ; with greater amplitude the drop 

 breaks up each time into two parts, which sometimes again flow to- 

 gether with surprising regularity, then extending in a direction at 



