M. Faye on Solar Repulsion. 287 



"by which, the mutual attraction of spherical bodies is determine d, 

 and a simple change of sign enables us to pass from a case of 

 attraction to one of repulsion. 



No one will deny the necessity for this narrow limitation .of 

 the sphere of activity assigned to molecular force, but must we 

 therefore conclude with Laplace that it is a special force, dis- 

 tinct from the great forces of nature which operate at all 

 distances ? No. It is easy to see that the repulsion due to 

 heat, and defined by its astronomical characters, exhibits 

 precisely the phenomena of forces with an insensible sphere 

 of activity, although in free space it operates at all distances. 

 That which conceals the true explanation, is that our minds, for 

 a long time habituated to speculations on Newtonian attraction, 

 experience a difficulty in considering forces of a totally differ- 

 ent nature, and if we speak of repulsion we conceive of it only 

 as an attraction with a change of sign, and philosophers like 

 Bessel only see a negative attraction in the repulsion so visibly 

 exerted by the sun. But it is not so ; solar repulsion as exhi- 

 bited in the movements and figures of comets, differs widely 

 from a negative attraction, first by its successive propagation, 

 and secondly, that it does not pass through matter as the attrac- 

 tive force does. It is in this last characteristic that we find the 

 key of the difficulty, and it is in harmony with all the evidence 

 collected in my researches, and on which I have had to insist 

 so often during the last three years. For if we consider the 

 essential character of the repulsive force we shall easily perceive 

 that it assumes in all bodies the conditions of a force with an 

 insensible sphere of activity. Each molecule of a body is in 

 fact surrounded, at an inappreciable distance, by other mole- 

 cules which receive its repulsive influence, and at the same time 

 behave to it like a screen. And as these molecules are not 

 mathematical points, and as their dimensions are considerable 

 when compared with the intervals which separate them, the " 

 repulsion due to heat — an action of surface, exhausting itself on 

 the siirface of the body which it affects — will find itself sensibly 

 reduced beyond the limits of the molecules surrounding each 

 centre of action. We may further conceive that the radius 

 of this boundary, that is to say, the sphere of activity of each 

 molecule, may be equal to a definite number of times the inter- 

 val between the several molecules, and thus, belonging to the 

 same order of minute magnitudes as they do, may be equally in- 

 appreciable. To this remark M. Faye adds in a note that instead 

 of being an absolute quantity this radius may depend upon tem- 

 perature, and he then observes : Thus the repulsive force which 

 acts at all distances in celestial spaces, finds itself reduced in the 

 interior of bodies to an action at insensible distances, and con- 

 sequently in all that concerns the mechanical action of heat, a 



