452 M. Gr. Van der Mensbruesre on the 



or> 



then be able to write, for the variation of the temperature, 



Kt-r. dS 

 dt= ^ 2 rj (Ibis) 



Tliis value shows that the less the mass m and the specific 

 heat k } the more will the temperature vary. 



Here are two curious facts, the explanation of which long 

 puzzled me, and which, in my provisional opinion, proceed 

 immediately from the above value of the variation of tempe- 

 rature dt : — 



(1) In a bubble of soap-solution or of liquid glycerine, as 

 is known, strange and altogether irregular motions are ob- 

 served very energetic when the bubble has just been blown, 

 and then less and less pronounced in proportion as the film 

 gets thinner. Now the insufflation of the air (supposed to 

 have the same temperature as the liquid) evidently produces a 

 larger and larger free surface ; and consequently the mass must 

 in its different points take temperatures as much lower as the 

 fresh surface supplied by each portion of the film is more 

 considerable. Owing to these differences of temperature, a 

 multitude of currents will arise, now in one direction, now in 

 the opposite. The continual descent of the liquid will main- 

 tain these phenomena for some time, until the extreme tenuity 

 of the film renders relative displacements of the liquid more 

 and more difficult and the motions less and less appreciable. 



(2) When the liquid film produced by placing a drop of 

 oil of turpentine upon the surface of distilled water has taken 

 the white tint of the first order, and upon this film a fresh 

 drop of turpentine is placed, this latter spreads out, like the 

 first, in a coloured layer, and the surrounding film thickens 

 as it is heaped up upon itself; soon the layer is arrested for a 

 few moments, and at length returns several millimetres 

 towards its centre. Is not this the effect, on the one hand, 

 of the spreading of the second drop, which cools its mass and 

 raises its superficial tension, and, on the other, of the collecting 

 upon itself of the surrounding film, a collecting which aug- 

 ments its temperature and diminishes its tension ? In this 

 way a slight difference of tension may be annulled, or even 

 change its sign ; and this would explain the fact which I 

 observed long since without at that time surmising the 

 cause. 



I could cite many other facts which appear to me intimately 

 connected with the cooling caused by variations of the free 

 surface of a liquid mass ; but I prefer to study first these 



