450 M. Van Marum on [Dec, 



But it would never have been found so difficult to account for 

 the repulsion of two bodies negatively electrified if attention had 

 been given to what has been incontestably demonstrated by more 

 recent observations ; and has been adopted by many philoso- 

 phers as a sound truth, and one well confirmed by experience^ 

 viz. that no electric force can arise from the surface of a body 

 unless this latter hein contact withbodies, either solid orjiuid, which 

 assume a contrary force, or be surrounded by such bodies. 



1. It necessarily follows from this universal law, already 

 demonstrated by Baptiste Beccaria in the Phil. Trans, for the 

 year 1770, vol. Ix. p. 277, that in the air surrounding a body to 

 which an electric force is communicated, there arises, for a cer- 

 tain distance, a contrary force. A body positively electrified and 

 placed in the air is, therefore, conformably to this law, sur- 

 rounded with an atmosphere of a contrary electric power ; and 

 a negatively electrified body is, on the other hand, surrounded 

 by a positively electrified atmosphere. 



2. It is evident that every electrified body must necessarily 

 be placed in the midst of this atmosphere of an opposite electric 

 force, the contrary being impossible for the following reasons. 

 Bodies possessing opposite electric forces reciprocally attract 

 each other ; consequently there exists an attraction between an 

 electrified body and its atmosphere ; namely, the electrified air 

 surrounding it, and possessing an opposite force. It follows 

 hence that the attractive forces which the electrified air exercises 

 on the bodies it surrounds must be equal on every side. Suppose, 

 for example, that the body A, fig. 2, placed in its electrified atmo- 

 sphere a, b, c, d, be retained by some cause towards the side d, 

 so that the breadth of the atmosphere on side b double that on 

 side c? ; it is then evident that the attractive force which the air 

 exercises on side b upon the body a will be much greater than 

 that on side d. This body, therefore, being drawn by unequal 

 forces, and in opposite directions, must, as soon as it is in its 

 power to yield to the attractive forces surrounding it, obey the 

 stronger attraction, and remove itself until it be drawn on 

 every side by attractions of equal power. Now this can hap- 

 pen only when it is in the centre of its electrified atmo- 

 sphere. 



3. When, therefore, an electric force is communicated to twa 

 light bodies, placed opposite to each other, and endued with 

 great mobihty, these two bodies will recede from each other 

 until each of them comes within the centre of the atmospheres in 

 which they have produced opposite forces. Suppose that the 

 bodies a, c, be electrified, each will be then surrounded by a 

 sphere of electrified air of contrary power. The moveable 

 bodies a, and c, will then not be able to retain themselves within 

 these atmospheres, one beside the other, as in figure 3, since 

 in this case they will be attracted by unequal forces towards b 

 and d ; of course they must then separate until each of them 



