o 



60 Of the Pi'opagation of Heat 



passing through it, impinge against the small solid 

 opaque body at the bottom of the vessel, and are there 

 absorbed^ must necessarily generate a certain quantity of 

 Heat ; a part of which will penetrate into the interior 

 parts of the solid, and a part of it wall be communicated 

 to those colder particles of the water which repose on its 

 surface. 



Let us suppose the quantity of Heat so communi- 

 cated to one of the integrant particles of the water to be 

 so small, that its effect in diminishing the specific gravity 

 of the particle is but just sufficient to cause it to move 

 upwards in the mass of the liquid with the very smallest 

 degree of velocity that would be perceptible by our or- 

 gans of sight, were the particle in motion large enough 

 to be visible. This would be at the rate of about one 

 hundredth part of an inch in a second. 



This velocity, though it appears to us to be slow in 

 the extreme, when we compare it with those motions that 

 we perceive among the various bodies by which we are 

 surrounded, yet we shall be surprised when we find what 

 a rapid succession of events it is capable of producing. 



If. we suppose the diameter of the integrant particles, 

 or molecules of water, to be 07ie millionth part of an inch 

 (and it is highly probable that they are even less *), 

 in that case, it is most certain that an individual particle, 

 moving on in a quiescent mass of that Fluid with the 

 velocity in question, namely, at the rate of ^-^q- part of 

 an inch in i second, would run through a space equal to 

 ten thousand times the length of its diameter in one second, 



* Leaf gold, such as is prepared and sold by the gold-beaters, is net four times as 

 thick as the diameter here assumed for the integrant particles of water. These 

 leaves of solid metal have been found by computation to be no more than -^-^^wq of 

 an inch in thickness. How much less must be the diameter of the integrant par- 

 ticles of gold ? 



