1819.] Pi'°f- Leslie on Ileal and Climate. 11 



their repulsions and attractions repeatedly fluctuate. But at ex- 

 ceedingly small distances, the action of light is uniformly attrac- 

 tive, as appears from its refraction. If the luminous particles in 

 their passage through a body be acted upon equally on every 

 side, their motion will not be affected : on the contrary, if they 

 happen to encroach within the limit due to any cluster of points, 

 their progress will, according to the degree of its proximity, be 

 either deranged or totally stopped. Hence the dispersion of iighi 

 observed both in mirrors and lenses, and hence too the diminu- 

 tion of its intensity occasioned by absorption. The distinction 

 of bodies into transparent and opaque is not founded in nature. 

 As no substance is perfectly transparent, so none is perfectly 

 opaque ; and the extremes are connected by an extensive inter- 

 mediate series. In the case of exceedingly thin plates, we are 

 obliged to admit the truth of the general proposition ;' but, in 

 thick masses, the quantity of light transmitted is so extremely 

 minute as to make no sensible impression on our oro-an of sio-ht. 

 In short, bodies approach to opacity, in proportion to their den- 

 sity, and the irregularity of their constitution. 



Whenever a body has a redundant quantity of heat, it makes 

 a copious emission of light ; which implies that there subsists 

 between the particles of this fluid a certain repulsion, propor- 

 tional most probably to the reciprocal of their distance. Such 

 at least is the conclusion into which we are drawn by the 

 analogy of the aeriform fluids. In these, however, the sphere of 

 repulsion has some palpable extent, and the joint actions of all 

 the included particles constitute the elasticity, which must be 

 proportional to the number of similar forces, or to the density. 

 But in the case of light, the repulsion most probably is confined" 

 to the adjacent particles : for the plane surface of a luminous body 

 emits rays equally in every direction,* nor is there any alteration 

 in this respect occasioned by giving a high polish ; a certain 

 proof that the limit of the repulsion between the particles of 

 light is incomparably nearer than that at which they are re- 

 flected from a mirror. A similar inference might be drawn from 

 the perfection of optical instruments, which could not have been 

 the case if the repulsion of the rays had any considerable extent. 

 The particles of light, however, must be vastly more distant 

 from, each other than those of other matter, since no increase of 

 weight has ever been observed to accompany the addition of 

 heat. 



The particles of light combined with a body endeavour to 

 distend, but are confined by the attraction of the other matter. 

 Upon every accession of heat, the particles of light must ap- 

 proach towards each other, and towards those of the body-; 

 both the repulsions, therefore, and the attractions, will receive 

 an augmentation, attended with a general expansion. If the 

 increase of the attraction of the luminous particles to the ad- 



• I have since proved that the intensity of the light or heat emitted from any 

 surface is proportional to the »ine of the inclination.— A. 



