202 Reflection and Refraction of Light. 



penetrate with rapidity each other's atmospherule, and becotue involved in 

 one; hence plainly, much of this ethereal matter constituting the separate 

 atmospherules will be released from the bond which held them so closely 

 together : and thus we have an adequate cause for the extrication and 

 propagation of light ; and this cause is such as agrees with all the phaeno- 

 mena in which it is concerned, with every chemical as well as optical fact. 



We may here remark, in a short digression, that this view of the subject 

 affords a complete and satisfactory explanation of the doctrine of definite 

 proportions in chemical combinations, and nothing can be imagined more 

 fully to enter into all the peculiarities of that fundamental law in chemistry, 

 and indeed in no otiier way can the important and interesting facts relating 

 to that doctrine be explained. 



20. During the passage of light, as thus stated, through free space, it is 

 manifest, from dynamical principles, that its motion will be uniform and in 

 a straight line : in traversing a medium of ethereal matter, its motion will 

 also be rectilinear, retaining its original velocity, because every retardation 

 of its sphere of repulsion will experience an equal acceleration ; but pulses 

 and undulations will necessarily be produced in such medium, through 

 which it moves. Let A, fig. 3, represent an ethereal atom moving in the 

 line, PC; the sphere of repulsion of the atom being denoted by the small 

 circle at A, the centre being at the middle : it is manifest, that this sphere 

 of repulsion will condense the ethereal matter in its path, and will therefore 

 be retarded ; but having passed the point of greatest condensation, it will 

 be equally accelerated, because the displaced ethereal atoms will return 

 behind the centre of the moving atom with the same force as that by 

 which they were displaced, on account of the perfect elasticity of the 

 ethereal matter, arising from the mutual repulsion of its atoms ; the points 

 of greatest condensation, and consequently of greatest retardation, are re- 

 presented at P, Q, &c., and those of greatest rarefaction and acceleration 

 at n, m, &c. The atom at A, is represented as having just passed the 

 point of greatest condensation, and that at B, as just beginning to en- 

 counter the greatest resistance ; the ethereal matter of the medium is 

 accelerating the motion of A, and retarding that of ^. In traversing 

 transparent media composed of tenacious atoms, since each of these atoms 

 will retain an atmospherule of ethereal matter, and all the pores and in- 

 terstices will be filled with this matter, similar phaenoraena of condensation 

 and rarefaction will occur : also the atoms of light, in passing the atmos- 

 pherules, will suffer small deflections, which will be continually compen- 

 sated in every part of its progress, when the medium is uniform in all 

 directions : hence, on the whole, the motion will still be uniform and recti- 

 linear; and pulses in this case also will be excited in the path, as above 

 described. Hence it follows that the velocity of light is the same in all 

 media, and in every direction of its course, the accelerations and retarda- 

 tions counterbalancing each other, as is more fully shewn in the treatise on 



