MR. GROVE ON THE ELECTRO-CHEMICAL POLARITY OF GASES. 
101 
bouring substances ; to show this effect well the needle should be positive and the 
plate negative, and the distance between them about an inch. 
I could dilate much further on these experiments, but have already trespassed 
perhaps too far for a Postscript. Variations in the form of the terminals, in the 
nature of the gas, vapour, or gaseous mixture, in the density of the gas, in the 
intensity and quantity of the discharge, in the nature of the plate, &c. will occur 
to those who may feel inclined to repeat these experiments, and if I am not over- 
sanguine, promise results of much interest. 
Description of Plate. 
PLATE VIII. 
Figures 1 to 10 show the spots and rings in the order referred to: it should be 
observed that printed figures give but a very imperfect notion of the actual 
effects. 
Fig. 1 1 is the coil apparatus, the contact breaker being in front. 
Fig. 12. The air-pump, of a construction which I proposed many years ago, and have 
found most useful for electrical or chemical experiments on gases. 
P. An imperforate piston, with a conical end, which, when pressed down, 
fits accurately the end of the tube, the apex touching the valve V, which 
opens outwards. 
A. Aperture for the air to rush from the receiver when the piston has been 
drawn beyond it. 
B. Bladder containing the gas to be experimented on. 
The piston-rod works air-tight in a collar of leathers, and the operation 
of the pump will be easily understood without further description. 
If it be required to examine the gas after experiment, a bladder, or 
tube leading to a pneumatic trough, can be attached at the extremity over 
the valve V. 
