14 Mr. M. M. P. Muir on Chemical Notation. 



mast be the motion of exceedingly small parts: hence arises 

 the conception of molecules, i. e. of small parts which remain 

 intact during the movements which each part makes when 

 the mass oi' the body to which it belongs is hot. 



28. Again, the molecular theory is in accord with known facts 

 concerning diffusion. If two gases be arranged in distinct 

 strata, it is found that in a short time the presence of that gas 

 which was at first confined entirely to the lower stratum is 

 perceptible in the upper stratum, and vice versa ; further, it is 

 found that if a stratum of gas be issuing through another gas 

 which is at rest, the movement of the former tends to commu- 

 nicate itself to the latter, which, in its turn, reacts upon the 

 moving gas, tending to bring it to rest. Again, if the upper 

 stratum of a gas be heated, it is found that the heat is commu- 

 nicated to the lower stratum. We have here, then, three 

 kinds of diffusion — diffusion of matter, diffusion of momentum, 

 and diffusion of energy. As we have learned to think of small 

 parts of a body in a state of motion, we may extend the idea 

 to the phenomena of diffusion, and imagine the little parts of 

 the two diffusing gases becoming mingled together ■ in the 

 diffusion of matter ; we may imagine the particles of the 

 moving stratum of gas passing upwards and downwards into 

 the surrounding strata of gas at rest, and hence having their 

 motion partly checked, while they in turn communicate some- 

 what of their motion to the particles of the surrounding gas ; 

 and we may imagine, finally, the communication of heat from 

 one part of a heated gas to another by the diffusion of the 

 heated particles of the gas. But the rates of these diffusions 

 are slower in liquids than in gases, especially the rate of the 

 diffusion of matter, which is slowest of all (if, indeed, it takes 

 place) in solids. Why is this? Because these little particles 

 or molecules (their existence being granted) have not in solids 

 the same freedom of motion which they have in liquids, in 

 which, again, they have less freedom of motion than in gases. 

 Although the particles of solids are not possessed of the same 

 freedom of motion as is granted to those of liquids, yet we can 

 easily see how there may be a rapid diffusion of energy from 

 particle to particle, and hence can understand the fact that 

 rapid diffusion of heat through solids is so often noticed. 



29. Again, the molecular theory is in accord with known 

 facts concerning evaporation and condensation. 



If we grant that a gas consists of a number of molecules in 

 a state of motion, we must believe that these molecules will 

 from time to time come into collision ; hence, even if their 

 initial velocity be the same, an inequality of velocity will be 

 produced. In a liquid the average velocity of the molecules 



