1213.  That I might be sure of the sensibility and action of the apparatus, I made such a change in one as ought upon principle to increase its inductive force, i.e.  I put a metallic lining into the lower hemisphere of app. i., so as to diminish the thickness of the intervening air in that part, from 0.62 to 0.435 of an inch:  this lining was carefully shaped and rounded so that it should not present a sudden projection within at its edge, but a gradual transition from the reduced interval in the lower part of the sphere to the larger one in the upper.

1214.  This change immediately caused app. i. to produce effects indicating that it had a greater aptness or capacity for induction than app. ii.  Thus, when a transferable charge in app. ii. of 469 deg. was divided with app. i., the former retained a charge of 225 deg., whilst the latter showed one of 227 deg., i.e. the former had lost 244 deg. in communicating 227 deg. to the latter:  on the other hand, when app. i. had a transferable charge in it of 381 deg. divided by contact with app. ii., it lost 181 deg. only, whilst it gave to app. ii. as many as 194:—­the sum of the divided forces being in the first instance less, and in the second instance greater than the original undivided charge.  These results are the more striking, as only one-half of the interior of app. i. was modified, and they show that the instruments are capable of bringing out differences in inductive force from amongst the errors of experiment, when these differences are much less than that produced by the alteration made in the present instance.

P iv. Induction in curved lines.

1215.  Amongst those results deduced from the molecular view of induction (1166.), which, being of a peculiar nature, are the best tests of the truth or error of the theory, the expected action in curved lines is, I think, the most important at present; for, if shown to take place in an unexceptionable manner, I do not see how the old theory of action at a distance and in straight lines can stand, or how the conclusion that ordinary induction is an action of contiguous particles can be resisted.

1216.  There are many forms of old experiments which might be quoted as favourable to, and consistent with the view I have adopted.  Such are most cases of electro-chemical decomposition, electrical brushes, auras, sparks, &c.; but as these might be considered equivocal evidence, inasmuch as they include a current and discharge, (though they have long been to me indications of prior molecular action (1230.)) I endeavoured to devise such experiments for first proofs as should not include transfer, but relate altogether to the pure simple inductive action of statical electricity.

1217.  It was also of importance to make these experiments in the simplest possible manner, using not more than one insulating medium or dielectric at a time, lest differences of slow conduction should produce effects which might erroneously be supposed to result from induction in curved lines.  It will be unnecessary to describe the steps of the investigation minutely; I will at once proceed to the simplest mode of proving the facts, first in air and then in other insulating media.