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



results of observation or experiment afford any indication of a 

 direct repulsive action exerted by radiant heat on the atoms 

 of bodies. It is universally admitted that radiant heat, when 

 imbibed by a body, acts as a repulsive or separating agency 

 among its molecules. It is also conceived that the conduc- 

 tion of heat is by radiation from atom to atom. The most 

 natural inference from these facts is that the waves of radiant 

 heat which pass from atom to atom directly urge the atoms away 

 from each other by repulsive impulses. Instead, however, of 

 adopting this simple idea, physicists have generally been in- 

 clined to refer the expansion of bodies from heat to some mode 

 of motion of the atoms originated by the heat received, though 

 no detailed satisfactory explanation has yet been given of the 

 manner in which such motions would directly originate an 

 expansion. Strangely enough, this notion is even entertained 

 by physicists who regard heat as the only cause of the repul- 

 sion subsisting among the molecules of bodies. It should here 

 be noted that if the expansion of all bodies of matter from 

 heat is to be ascribed to a direct impulsive or repulsive action 

 of heat-waves proceeding from one atom and falling upon the 

 surrounding atoms, then this force takes effect at the greatest 

 distances by which the atoms are separated in the rarest gas 

 under the feeblest pressure ; and we should thus be led to ex- 

 pect that heated bodies in contact with each other might ma- 

 nifest signs of repulsion. 



In point of fact many evidences of a heat-repulsion subsist- 

 ing between particles of different bodies in contact or in close 

 proximity have been adduced by different experimentalists and 

 writers on physics, some of which may be briefly mentioned. 



1. "When pure silica in an extreme state of division is 

 highly heated, the slightest motion then causes the particles of 

 the powder to slide over each other, and the surface of the 

 powder is thrown into undulations almost like those of a 

 liquid." 



2. A rise of temperature is attended with a decrease of capil- 

 lary attraction. Also the frictional resistance to the flow of 

 water in pipes is diminished by heat. 



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 closely 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 pro- 

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

 by Professor Forbes, of Edinburgh, in opposition to Faraday, 



* Miller's f Physics/ p. 285. 



