DIFFUSION. 



other whan they are once mixed If, however, the molecules of one portion 

 hare any property whereby they can be distinguished from those of the other, 

 then that property will be communicated from one part of the medium to an 

 adjoining part, and that either by convection that is by the molecules them- 

 selves patting out of one part into the other, carrying the property with 

 then or by tranamisaion that is by the property being communicated from 

 one molecule to another during their encounters. The chemical properties by 

 which different substances are recognized are inseparable from their molecules, 

 o that the diffusion of such properties can take place only by the trans- 

 ference of the molecules themselves, but the momentum of a molecule in any 

 given direction and its energy are also properties which may be different in 

 different molecules, but which may be communicated from one molecule to 

 another. Hence the diffusion of momentum and that of energy through the 

 medium can take place in two different ways, whereas the diffusion of matter 

 can take place only in one of these ways. 



In gases the great majority of the particles, at any instant, are describing 

 free paths, and it is therefore possible to shew that there is a simple numerical 

 relation between the coefficients of the three kinds of diffusion.-^the diffusion 

 of matter, the lateral diffusion of velocity (which is the phenomenon known as 

 the internal friction or viscosity of fluids), and the diffusion of energy (which 

 is called the conduction of heat). But in liquids the majority of the molecules 

 are engaged at close quarters with one or more other molecules, so that the 

 transmission of momentum and of energy takes place in a far greater degree 

 by communication from one molecule to another, than by convection by the 

 molecules' themselves. Hence the ratios of the coefficient of diffusion to those 

 of viscosity and thermal conductivity ard much smaller in liquids than in gases. 



Theory of the Wet Bulb Thermometer. 



The temperature indicated by the wet bulb thermometer is determined in 

 great part by the relation between the coefficients of diffusion and thermal 

 conductivity. As the water evaporates from the wet bulb heat must be sup- 

 plied to it by convection, conduction, or radiation. This supply of heat will 

 not be sufficient to maintain the temperature constant till the temperature of 

 the wet bulb has sunk so far below that of the surrounding air and other 



