METHODS OF ELECTRICAL STIMULATION 13 



are assumed by Clausius and Maxwell to be in 

 constant rapid motion in all directions. It is 

 this motion which causes the "disappearance" 

 of a gas " set free " in the open air. The mole- 

 cules of air are also in rapid motion and fre- 

 quently collide with each other and with those 

 of the escaping gas, but the air molecules are 

 far from filling the space in which they move 

 and the molecules of gas rapidly pass off be- 

 tween them. In their flight, they strike with 

 a measurable force whatever opposes them, be 

 it another flying molecule or some boundary 

 wall. The force of the blow is the "pressure" 

 of the molecule. If the gas be confined by an 

 impermeable wall, that is, a wall the inevitable 

 openings in which are too small for the gas 

 molecules to pass, the sum of the blows of the 

 molecules dashing against this wall will be the 

 total pressure of the gas. 



Partial pressure. If the molecules of a second 

 gas be within the containing vessel, they also 

 will strike the boundary. The force of their 

 blows will be entirely independent of the force 

 of the blows struck by the molecules of the 

 first gas. It is as if black and white balls were 

 thrown at the same time against a wall; their 

 blows would be independent of each other. 



The molecules of gas and the confining walls 



