460 Prof. J. C. Maxwell on Molecules. 



and below will carry their horizontal motion with them, and so 

 tend to communicate motion to the neighbouring strata, while 

 molecules diffusing out of the neighbouring strata into the mo- 

 ving one will tend to bring it to rest. The action between the 

 strata is somewhat like that of two rough surfaces, one of which 

 slides over the other, rubbing on it. Friction is the name given 

 to this action between solid bodies ; in the case of fluids it is 

 called internal friction, or viscosity. 



It is, in fact, only another kind of diffusion — a lateral diffu- 

 sion of momentum ; and its amount can be calculated from data 

 derived from observations of the first kind of diffusion, that of 

 matter. The comparative values of the viscosity of different 

 gases were determined by Graham in his researches on the trans- 

 piration of gases through long narrow tubes ; and their absolute 

 values have been deduced from experiments on the oscillation of 

 disks by Oscar Meyer and myself. 



Another way of tracing the diffusion of molecules through 

 calm air is to heat the upper stratum of the air in a vessel, and 

 to observe the rate at which this heat is communicated to the 

 lower strata. This, in fact, is a third kind of diffusion — that of 

 energy ; and the rate at which it must take place was calculated 

 from data derived from experiments on viscosity before any 

 direct experiments on the conduction of heat had been made. 

 Professor Stefan, of Vienna, has recently, by a very delicate 

 method, succeeded in determining the conductivity of air ; and 

 he finds it, as he tells us, in striking agreement with the value 

 predicted by the theory. 



All these three kinds of diffusion (the diffusion of matter, of 

 momentum, and of energy) are carried on by the motion of the 

 molecules. The greater the velocity of the molecules and the 

 further they travel before their paths are altered by collision 

 with other molecules, the more rapid will be the diffusion. Now 

 we know already the velocity of the molecules ; and therefore, by 

 experiments on diffusion, we can determine how far, on an 

 average, a molecule travels without striking another. Professor 

 Clausius, of Bonn, who first gave us precise ideas about the 

 motion of agitation of molecules, calls this distance the mean 

 path of a molecule. I have calculated, from Professor Lo- 

 schmidt's diffusion-experiments, the mean path of the mole- 

 cules of four well-known gases. The average distance travelled 

 by a molecule between one collision and another is given in the 

 Table. It is a very small distance, quite imperceptible to us 

 even with our best microscopes. Roughly speaking, it is about 

 the tenth part of the length of a wave of light, which you know 

 is a very small quantity. Of course the time spent on so short 

 a path by such swift molecules must be very small. I have cal- 



