Transactions of the Cambridge PJiilosophical Society* 69 



'wMch value, in the case of reflection from "water at the proper angle, 

 is YjY Pfi'^t of the intensity of the incident wave. This minimum 

 value increases rapidly as the index of refraction increases ; and 

 thus the quantity of light reflected at the polarizing angle becomes 

 considerable for highly refracting substances ; a fact, v^hich has 

 been long known to experimental philosophers. 



In J\Ir. Earnshaw's memoir the retherial medium is treated as a 

 system of detached particles ; and he is led by his investigations 

 to various conclusions, of which the most important are, that the 

 molecular forces which regulate the vibrations of the aether do 

 not vary according to Newton's law of universal gravitation, but 

 that these forces are repulsive, and vary according to an inverse 

 power of the distance greater than two. M. Cauchy, in his "Mi- 

 moire sur la Dispersion de la Liimiere," had inferred from his ana- 

 lysis that " in the neighbourhood of contact, the action of two 

 particles is repulsive, and reciprocally proportional to the fourth 

 power of the distance." 



Mr. Kelland, in a memoir contained in the previous volume of 

 these Transactions, (vol. vi. Part I. p. 178) had been led by calcu- 

 lations founded upon the phsenomena of the dispersion of light, to 

 conclude that the particles of the tether act on each other with 

 forces varying inversely as the square of the distance. We shall not 

 here pretend to discuss the diff"erence of the results thus obtained 

 by these two mathematicians. But we must notice Professor Kel- 

 land's memoir " On Molecular Equilibrium," contained in the vo- 

 lume now under our notice. In this notice Mr. Kelland pursues a 

 train of speculation somewhat similar to that employed in the last 

 centur)'' by Dr. Knight, in his " Attempt to explain all the phccno- 

 mena of Is^ature by two principles, attraction and repulsion;" and 

 by Boscovich, in his " Theory of Natural Philosophy reduced to a 

 single law of the forces which exist in Nature." Mr. Kelland states 

 his assumption as follows : "I purpose to commence my investiga- 

 tion by retaining M. Mosotti's hypothesis of two systems of parti- 

 cles*, repulsive towards atoms of their own kind, but each respect- 

 iA'ely attractive towards the atoms of the other. We will call one 

 system of particles caloric, and the other matter." He then adds the 

 other suppositions by which these two elements are distinguished 

 from each other ; the atoms of caloric are distributed through 

 space, the atoms of matter occupy only given positions. In both the 

 density will vary from point to point ; but the particles of matter 

 are supposed to be much more widely separated than the particles 

 of caloric ; so that a material particle may be considered as a nu- 

 cleus about which the particles of caloric are collected, forming its 

 atmosphere. But Mr. Kelland afterwards determines the conditions 

 of equilibrium of a system in which the atoms of caloric are re- 

 pulsive of those of matter ; and on the same hypothesis he deter- 

 mines the mutual action of two particles of matter, together with 

 the caloric surrounding them. For the general relations between 



* Taylor's Scientific Memoirs, vol. i. p. 448, and L. and E. Phil. Mag. 

 vol. X. p. 320. 



