442 Mr. Q-. J. Stoney on the Penetration of 



occasionally produced by dropping the spirits of wine upon 

 water. And every one is familiar with them when, in some 

 states of the weather, they roll about in numbers on allowing 

 water to drip from an oar upon the sea. Recently they were 

 abundantly produced by splashing the water of a neighbouring 

 pond; and I took advantage of the opportunity to ascertain 

 that the conditions required by the hypothesis were fulfilled. 

 The temperature of the air was about 1 5°, that of the surface 

 of the water 18J°; and a very dry breeze was blowing, which 

 so facilitated evaporation from the drops * that they probably 

 maintained a temperature as low as 10°, a temperature which 

 my thermometer reached when I left a damp weed in contact 

 with one side of the bulb. 



When globules of methylated spirit were formed upon a 

 beaker nearly full of the same liquid, which was progressively 

 warmed, it was found that, when the air was still, there was a 

 particular temperature at which the drops were most persistent. 

 At this temperature some lasted for as long as twelve or four- 

 teen seconds. As the temperature rose beyond this point, the 

 atmosphere of vapour impeded evaporation and the persistence 

 of the drops became less ; but by gently blowing on the sur- 

 face so as to accelerate the evaporation, it was found possible 

 to keep some of the smaller drops in existence for two or three 

 minutes j, during which time they very slowly dwindled away 

 till they were very minute, and then suddenly vanished. When 

 the temperature of the beaker full of spirit was allowed to fall 

 below the point above referred to, the duration of the drops 

 also became progressively less ; but they could still be formed, 

 though short-lived, at a temperature a little below that of the 

 room. 



From this, and from the circumstance that I succeeded in 

 forming some within a bottle of methylated spirit which had 

 been standing open for a while, and within which evaporation 

 must have been feeble, it is evident that a drop can be sup- 

 formed can be fed from the pipette and so greatly augmented in size. 

 When in this way the drop is made so large as to be very much flattened 

 out, tremulous motions arise similar to those which Bouligny observed in 

 his spheroidal drop, but less violent. 



* The liquid on which the drops rest is no doubt also cooled by evapo- 

 ration, but in a trifling degree, because convection currents constantly 

 bring to the surface an accession of wann liquid from below. 



t In four better-arranged experiments, since made in the laboratory of 

 the Royal Dublin Society by Mr. R, J. Moss and myself, and of which 

 we hope to give an account to the Society, we succeeded in maintaining 

 similar drops of larger size formed on methylated ether for ten, fourteen, 

 sixteen, and twenty-six minutes, respectively ; and we believe that it will 

 not be difficult, by securing a greater constancy in the conditions indicated 

 by the theory, to prolong their existence very much more. 



