W. A. Norto7i — Physical Constitution of (he Sun. 401 



3. " The spheroidal state of liquids is a complicated result 

 of four distinct causes. The most influential is the repulsive 

 force which heat exerts between objects which are closel}' ap- 

 proximated to each other."* 



4. The vibrations of heated metals, as shown in " Trevilyan's 

 Instrument," or "rocker," resting on a block of metal, are prob- 

 ably due to the direct repulsive force of heat, as maintained 



) the sudden 



pansion by heat of the body on which the rocker rests. Fara- 

 day's explanation of these curious phenomena is adopted by 

 Tpdall, who endeavors to overthrow Prof Forbes's theory. 

 His experiments serve to disclose the fallacy of certain fea- 

 tures 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 discussion of the results of the experiments. 



Let us now consider if any sufficient evidence exists of a 

 general force of molecular repulsion in operation at all tempera- 

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

 Y^ iieed, in fact, to look no farther for this than to the simple 

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

 teuce between them, minute as it is, much exceeds the ranee of 

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

 weight of the upper body is counteracted bv a repulsion be- 

 tween the molecules about the point of contact Dr. Kobison 

 iias shown in his 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 

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

 njsth 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 maybe made 

 ^"h regard to all cases of the apparent contact of homogene- 

 ous substances under a moderate pressure. It is only bv m- 

 cfeasing the pressure more or less, that the contiguous particles 

 can be brought within the range of their reciprocal attraction 

 Jt cohesion. When the particles are readily displaced among 

 flemselves under the direct action of a pressure, or blow, as m 

 rnLT^ Z^ ^^ft or malleable substances, a permanent union 

 pay be effected without difficulty between the surfaces: that 

 COT., f/'*^^ repulsion of some of the particles may be over- 

 r^T' i^'"' attraction of cohesion brought into play at the 

 '^^ced distance, and an equilibrium established at the neutral 

 » MUler's Physics, p. 285. 



