34?* Mr. J. Barton on the Physical Causes 



4. When the coefficients of the attractive and repulsive 

 forces bear a certain relation to each other, and to the original 

 velocity of approach, the orbit described will resemble a con- 

 choid^ its two branches being ultimately in one and the same 

 line. In this case the body is diathermanous, the rays of heat 

 passing through it apparently in a straight course. The course 

 of a ray through a solid body can never be in fact straight, 

 unless its velocity be infinite ; but in proportion as the velo- 

 city approaches this limit, the ray will experience less disturb- 

 ance in passing through bodies imperfectly diathermanous. 

 And accordingly it is found, that rays from incandescent bodies 

 pass through some media which do not admit the passage 

 of rays issuing from bodies of lower temperature, the velocity 

 of projection evidently depending on the repulsive force ; and 

 this, in its turn, on the temperature. 



5. It must not however be supposed that opake or adiatlier- 

 manous bodies are impervious to the rays of heat. They differ 

 from transparent or diathermanous bodies only in this, that the 

 entering ray, instead of passing through immediately, and 

 escaping from the opposite side, is entangled in a circuitous 

 and irregular course among the particles, until by chance 

 it reaches the surface, when it escapes, unless the angle of its 

 direction with the tangent be so small that it is drawn back 

 again by the attractive force. A particle of heat is therefore 

 detained much longer in passing through an opake than 

 through a transparent body, and contributes much more to 

 raise its temperature, and hence it also appears why the pas- 

 sage of heat through opake bodies is very much slower than 

 through transparent media. 



6. The particles of solid bodies are drawn towards each 

 other by two forces, their mutual attraction, and their attrac- 

 tion for the atmospheres of heat about the others. They are 

 also kept apart by two forces, the mutual repulsion of those 

 atmospheres of heat, and the repulsion of the particles of free 

 or sensible heat, which happen at the time to be passing be- 

 tween them. As long as the number of these last remains 

 unchanged, the balance of the opposing forces is a balance of 

 stable equilibrium, in as much as the attractive force dimi- 

 nishes with the distance in a less ratio than the repulsive 

 force. If the number of particles of sensible heat is increased, 

 the body dilates till the equilibrium is restored ; but if this 

 dilatation proceed to a certain point, the orbits of a cer- 

 tain number of the revolving particles of heat will be changed 

 from the hyperholoidal to the ellipsoidal form ; that is to say, 

 a certain quantity of sensibly heat will be converted into latent 

 heat. At the same time another portion of sensible heat will 

 be acquired from the surrounding bodies and the united re- 



