Prof. J. C. Maxwell on Molecules. 459 



lighter gas being placed above the heavier, so as to avoid the 

 formation of currents. He then opened a sliding valve so as to 

 make the two tubes into one ; and after leaving the gases to 

 themselves for an hour or so, he shut the valve, and determined 

 how much of each gas had diffused into the other. 



As most gases are invisible, I shall exhibit gaseous diffusion 

 to you by means of two gases (ammonia and hydrochloric acid) 

 which when they meet form a solid product. The ammonia, 

 being the lighter gas, is placed above the hydrochloric acid with 

 a stratum of air between ; but you will soon see that the gases 

 can diffuse through this stratum of air and produce a cloud 

 of white smoke when they meet. During the whole of this 

 process no currents or any other visible motion can be de- 

 tected. Every part of the vessel appears as calm as a jar of un- 

 disturbed air. 



But, according to our theory, the same kind of motion is 

 going on in calm air as in the interdiffusing gases, the only dif- 

 ference being that we can trace the molecules from one place to 

 another more easily when they are of a different nature from 

 those through which they are diffusing. 



If we wish to form a mental representation of what is going 

 on among the molecules in calm air, we cannot do better than 

 observe a swarm of bees, when every individual bee is flying 

 furiously, first in one direction and then in another, while the 

 swarm, as a whole, either remains at rest or sails slowly through 

 the air. 



In certain seasons swarms of bees are apt to fly off to a great 

 distance, and the owners, in order to identify their property 

 when they find them on other people's ground, sometimes throw 

 handfuls of flour at the swarm. Now let us suppose that the 

 flour thrown at the flying swarm has whitened those bees only 

 which happened to be in the lower half of the swarm, leaving 

 those in the upper half free from flour. If the bees still go on 

 flying hither and thither in an irregular manner, the floury bees 

 will be found in continually increasing proportions in the upper 

 part of the swarm till they have become equally diffused through 

 every part of it. But the reason of this diffusion is not because 

 the bees were marked with flour, but because they are flying 

 about. The only effect of the marking is to enable us to iden- 

 tify certain bees. 



We have no means of marking a select number of molecules 

 of air, so as to trace them after they have become diffused among 

 others, but we may communicate to them some property by 

 which we may obtain evidence of their diffusion. 



For instance, if a horizontal stratum of air is moving horizon- 

 tally, molecules diffusing out of this stratum into those above 



