366 Prof. Forbes on the Refraction and Polarization of Heat, 



combination." Unless the particles of oxygen were elastic or 

 repulsive as respects the contiguous particles of hydrogen, 

 elasticity could not be a force opposed to affinity, and the di- 

 minution of elasticity or repulsion by the action of the plate, 

 could not determine the union of oxygen with hydrogen *. 

 Manchester, April 7, 1835. 



LIX. On the Refraction and Polarization of Heat, By 

 James D. Forbes, Esq., F.R.SS. L. <^jE., Professor of Na- 

 tural Philosophy/ in the University of Edinburgh, 



[Continued from p. 291, and concluded.] 



66. T^HE table generally points to a coincidence, and that as 

 -■• close as by the nature of the experiments we should 

 perhaps be warranted in expecting. If there be any excess in the 

 second column of results (which the observations with incan- 

 descent platinum might lead us to suspect), it is more than 

 probable that it arises from some imperfection in the appara- 

 tus employed, such as the incomplete parallelism or perpen- 

 dicularity of the mica plates employed to polarize, a circum- 

 stance which was not minutely attended to. 



67. The result, however, is highly satisfactory, as indica- 

 ting the almost exactly complementary nature of the ordinary 

 and extraordinary pencils, as in light. 



68. The somewhat complicated conditions of the variable 

 intensities of the ordinary and extraordinary images (which it 

 is to be recollected correspond to the Parallel and Perpendi- 

 cular positions of the analysing plate) in the case of light, are 

 easiest kept in mind by Fresnel's formulae. 



02= F^l 1 _ giii^ 2 i sin^ TT (^'"- ^) Y 



E2=F24^sin2 2/sin2 7r ^^""-A^ 



where 0^ E^ and F-, have the same signification as in (64), 

 and / represents the angle between the plane of polarization 

 and the principal plane of the crystal : o — ^ is the difference 



* In proof that the repulsion existinjr between unlike gaseous molecules 

 is a force opposed to chemical union, it is worthy of remark, that of such 

 gases as combine spontaneously, when simply mingled, one or both are 

 generally found among that class which have been reduced to a liquid 

 form, and in which the repulsive force between the constituent molecules 

 is therefore least energetic. 



2 ( 2 X I T 



t This corresponds to the formula ^sin^ 2 (5 1 1 — cos —^ | of 



Airy*s Tract on the Undulatonj Theory, Art. 172. Both are only restricted 

 expressions of more general theorems. 



