Prof. W. A. Norton on the Physical Constitution of the Sun. 61 



who conceived that they were attributable to the sudden expan- 

 sion by heat of the body on which the rocker rests. Faraday's 

 explanation of these curious phenomena is adopted by Tyndall, 

 who endeavours to overthrow Professor Forbes's theory. His 

 experiments serve to disclose the fallacy of certain features of 

 the theory, but do not dislodge the fundamental idea that the 

 phenomena are due to a force of heat-repulsion. This might 

 be made apparent if we had space for a statement of the general 

 principles on which the explanation rests, and for a detailed dis- 

 cussion of the results of the experiments. 



Let us now consider if any sufficient evidence exists of a ge- 

 neral force of molecular repulsion in operation at all tempera- 

 tures, beyond the sphere of sensible adhesion or cohesion. We 

 need, in fact, to look no further for this than to the simple 

 fact that in the ordinary contact of bodies the interval of dis- 

 tance between them, minute as it is, much exceeds the range of 

 the attraction of cohesion or adhesion ; for in such contact the 

 weight of the upper body is counteracted by a repulsion between 

 the molecules about the point of contact. Dr. Robinson has 

 shown, in his c System of Mechanical Philosophy/ that if two 

 glasses, one slightly convex the other flat, are placed on each 

 other and pressed by a force of 1000 pounds to the square inch, 

 they are still at the distance from each other of the thickness 

 of the top of a soap-bubble just before it bursts, or at least 

 44*5(j of an inch. In effecting this contact there was no evi- 

 dence of any attraction existing at distances greater than that 

 at which the contact occurred. A similar remark may be made 

 with regard to all cases of the apparent contact of homogeneous 

 substances under a moderate pressure. It is only by increasing 

 the pressure more or less that the contiguous particles can be 

 brought within the range of their reciprocal attraction of cohe- 

 sion. When the particles are readily displaced among them- 

 selves under the direct action of a pressure or blow, as in the 

 case of soft or malleable substances, a permanent union may be 

 effected without difficulty between the surfaces; that is, the 

 outer repulsion of some of the particles may be overcome, their 

 attraction of cohesion brought into play at the reduced distance, 

 and an equilibrium established at the neutral point between this 

 attraction and the inner repulsion. This occurs in the welding 

 of iron. 



Other evidences of an effective repulsion in operation between 

 the molecules of bodies in contact, or in close proximity, are 

 cited in treatises on physics, although it is not always distinctly 

 recognized that the sphere of its action lies entirely without that 

 of the effective molecular attraction. Now, what is the range of 

 this effective repulsion between bodies ? It obviously extends 



