Induction in curved Lines — in Fluids^ Solids. 359 



could not be found on the top of the hemisphere. Removing 

 this matter the hnes of force resumed their former direction. 

 The experiment affords proofs of the lateral tension of these 

 lines, and supplies a warning to remove such matter in re- 

 peating the above investigation. 



1226. After these results on curved inductive action in air 

 I extended the experiments to other gases, using first carbonic 

 acid and then hydrogen : the phaenomena were precisely 

 those already described. In these experiments I found that 

 if the gases were confined in vessels they required to be very 

 large, for whether of glass or earthenware, the conducting 

 power of such materials is so great that the induction of the 

 excited shell-lac cylinder towards them is as much is if they 

 were metal ; and if the vessels be small, so great a portion of 

 the inductive force is determined towards them that the lateral 

 tension or mutual repulsion of the lines of force before spoken 

 of, (1224.) by which their inflexion is caused, is so much re- 

 lieved in other directions, that no inductive charge will be 

 given to the carrier ball in the positions A-, I, m, n, o^p, (fig. 7.). 

 A very good mode of making the experiment is to let large 

 currents of the gases ascend or descend through the air, and 

 carry on the experiments in these currents. 



1227. These experiments were then varied by the substitu- 

 tion of a liquid dielectric, namely, oil of turipentine^ in place 

 of air and gases. A dish of thin glass well covered with a 

 film of shell-lac, (1272.) and found by trial to insulate well, 

 had some highly rectified oil of turpentine put into it to the 

 depth of half an inch, and being then placed upon the top of 

 the brass hemisphere, (fig. 7.) observations were made with 

 the carrier ball as before (1224.). The results were the same, 

 and the circumstance of some of the positions being within 

 the fluid and some without, made no sensible difference. 



1228. Lastly, I used a few solid dielectrics for the same 

 purpose, and with the same results. These were shell-lac, 

 sulphur, fused and cast borate of lead, flint glass well covered 

 with a film of lac, and spermaceti. The following was the 

 form of experiment with sulphur, and all were of the same 

 kind. A square plate of the substance, two inches in extent 

 and 0*6 of an inch in thickness, was cast with a small hole or 

 depression in the middle of one surface to receive the carrier 

 ball. This was placed upon the surface of the metal hemi- 

 sphere (fig. 9.) arranged on the excited lac as in former cases, 

 and observations wei"e made at w, o, /?, and q. Great care 

 was required in these experiments to free the sulphur or other 

 solid substance from any charge it might previously have 

 received. This was done by breathing and wiping (1203.), 



