194 



LORD KELVIN, DR. M. MACLEAN, AND MR. A. GALT, ON THE 



movement of the piston begun. Usually about thirteen seconds were required to 

 make these changes. 



Fig. 2. 



22. When the electrified air inside the can was expelled by dropping the piston 

 to the bottom, the reading of the electrometer went off the scale, and a shorter drop 

 had to be used to get a convenient deflection. 



23. A drop of 11 '5 centims., by which 3979 cub.- centims. of air were expelled, 

 gave a deflection of 1060 scale divisions. The quantity of electricity on the can and 

 connecting wire and insulated pair of quadrants of the electrometer which gives this 

 deflection, was (by the method of 18) found to be 1'47 C.G.S. electrostatic. This, 

 therefore, was the quantity of electricity of the opposite kind in the 3979 cub. 

 centims. of air expelled from the open bottom of the can ; and the electric density of 

 this air was therefore 3'7 X 10~ l C.G.S. electrostatic per cubic centimetre. 



24. In preliminary experiments before electrifying the air inside the can by 

 needle point, it was found that the dropping of the piston produced no deflection on 

 the electrometer. 



