Propagation of Heat in Liquids. 285 



munication of heat in this liquid, from particle to par- 

 ticle, de proche en proche, is impossible. They do not 

 even prove that heat did not descend, to a small distance^ 

 below the level of the end of the wooden tube in these 

 experiments ; for it is certain that that event could take 

 place without the thermometer, which was situated a 

 little lower, being in any way affected by that heat. 



The particles of water situated at a very small distance 

 below the level of the lower end of the wooden tube, 

 being heated by the stratum of hot water which rested 

 immediately on them, might have been displaced by the 

 surrounding colder and denser particles, and forced to 

 rise to the surface ; and these last being in their turn 

 heated, forced upwards and replaced by other cold par- 

 ticles, it is evident that the heat could not make its 

 way downwards so far as to arrive at the thermometer 

 through a stratum of liquid, which, though apparently 

 at rest, was nevertheless in part composed of particles 

 which were continually changing. 



I have long suspected that the apparent impossibility 

 of a direct communication of heat between neighbouring 

 particles of fluids depends solely on the great mobility 

 of those particles (see note, p. 202, Vol. II. of my 

 Essays, jd edition, London, 1800); and if this sus- 

 picion be well founded, it is certain that when this 

 mobility ceases, the effect which depends on it must 

 cease likewise. 



When I speak of the mobility of the particles of a 

 liquid amongst each other, I am very far, as I have 

 already observed, from supposing that individually they 

 can enjoy a free motion. I was formerly of that opinion, 

 but a more attentive investigation of the phenomena has 

 convinced me that I was mistaken. But although one 



