562 fASSAGE OF HEAT THROUGH FLUIDS. 



meter through a. stratum of liquid, which though apparentl}" at 



rest, was nevertheless in part composed of particles which 



were continually changing. 



Opinion of the I have long suspected that the apparent impossibility of a 



author that the jjj.g^t communication of heat between neighbouring particles 

 imperfect eon- = '=' ^ 



ducti,i,£t power of fluids depends solely on the great mobihty of those parti- 



of fluids IS ^jp„ /ggg ^ j^^j.^ 202, torn. ii. of my Essays, third edition, 



owing to the ^ ' i ' _ •' *' ' _ / 



mobility of London, 1800); and if this suspicion be well founded, it is 

 their parts. . ccj-t^in that when this mobility ceases, the eflect which depends 



on it must cease likewise. 

 . Notoftlieir When I speak of the mobility of the particles of a liquid 



particles J but amongst each other, I am very far (as I have already observ- 

 of sensible por- ° _ J \ j 



tions or parts, ed) from supposing that individually they can enjoy a free mo- 

 tion. I was formerly of that opinion, but a more attentive in- 

 vestigation of the phenomena has convinced me that I was 

 mistaken. But although one individual particle of a liquid can 

 never be put in motion in consequence of a change of its spe- 

 cific gravity occasioned by a change of temperature, yet what 

 cannot happen to a single particle, may easily, and must ne- 

 cessarily, happen to small masses of the liquid consisting of a 

 great number of these particles united ; as is abundantly 

 proved by the currents which are so easily excited by the con- 

 tact of a hot or cold body plunged in a liquid. 

 The particles The force by which the particles of liquids adhere together 



of fluids-have -g very great: and it is more than probable that it is the cause 

 much adhesion. . "^ ° , ^ , 



of many very mterestmg phenomena, and amongst others, of 



the suspension of the heavy bodies which much lighter liquids 

 so frequently hold in solution. 

 Jlstimate of its From the result of an experiment which I made some years 

 quantity. ^^^^ -^^ order to determine the measure of the viscosity, or the 



want of perfect fluidity in water, at the temperature of 64" 

 Farenh. I found reason to conclude, that a solid body having a 

 •iirface equal to 36S square inches, which should weigh 

 only one grain troy more than an equal volume of water, would 

 remain suspended in that liquid; and from this datum it is 

 easy to find by calculation, what ought to be the diameter of 

 a small solid spherule of the heaviest matter, of gold for in- 

 stance, in order to its remaining suspended in water in conse- 

 quence of the viscosity of that liquid. 



Having 



