The Constitution of Matter 79 



result in the complete admixture of the bromine and 

 air with one another, a result which our observations 

 have shown us is actually achieved, although the 

 density of bromine is more than 5 times that of air. 

 A precisely similar explanation of the phenomenon 

 of liquid diffusion is afforded by the Kinetic Hypothesis. 



Exercise for Student. 



Suggest two reasons, based on the Kinetic Hypothesis, for gaseous 

 diffusion being a more rapid process than liquid diffusion. 



The above is naturally a very incomplete presenta- 

 tion of the Kinetic Hypothesis, and one other point 

 deserves attention. The molecules of solids, liquids, 

 and gases are not assumed to be all of similar size and 

 mass. Thus the molecules of water are supposed to 

 be larger than those of steam (= gaseous water) and 

 we have some evidence that they are 3 to 4 times as 

 large. On the other hand, the molecules of water are 

 probably considerably smaller than those of ice (= solid 

 water). 



46. Many interesting experiments may be per- 

 formed which lend support to the idea of the existence 

 in gaseous substances of molecules moving at rates 

 which depend upon the density of the gas, the slower 

 molecules being those of the heavier gases. One such 

 experiment will next be described. 



EXPEKIMENT 26. Procure a glass tube about 18" 

 long and of J" bore. Make a cream of plaster of 

 Paris and water in a shallow vessel, such as a saucer, 

 and dip the end of the tube into this cream to a depth 

 of about J". Fix the tube in an upright position with 

 its end still in the cream, by means of a clamp and 

 retort stand, and leave it till the plaster has set. Re- 

 move it and carefully cut away the excess of plaster 



