484 Mr. E. H. Cook on the 



action of (ho molecules of material bodies upon the aether, an 

 addition which makes a greater strain upon the imagination. 

 It is necessary not only to assume that the proximity of the 

 lother-partieles to those of the crystal causes a difference in 

 the elasticity of the aether, but also that there is no " abstract 

 geometrical relation " between the shape of the crystal and 

 the axes of elasticity produced by it in the aether. Now, 

 as it is certain that the arrangement of the molecules of the 

 crystal in the particular shape gave rise to the axes of elasti- 

 city in the aether, we are shown a body whose shape produces 

 a certain result, which result bears no relation to the shape 

 that produced it. If the imprisonment of the aether between 

 the molecules of a body produces axes of elasticity in that 

 aether, then surely these axes must bear some close geome- 

 trical relation to the shape in which the molecules are 

 arranged. 



3. Optical Properties of Carbon Compounds. 



The researches of Gladstone and Dale, of Landolt, and, more 

 recently, of Briihl, have opened up a field of inquiry which 

 cannot but be productive of important results. The influences 

 exerted by organic compounds upon light which passes through 

 them are of two kinds, viz. (1) simple refraction of a ray, and 

 (2) the turning of the plane of polarization. Let us briefly 

 examine these separately. 



1. Refraction of Organic Compounds. — From a series of 

 observations made by Dulong it has been shown that not only 

 does the density of a body change with the temperature, but 

 also that the refractive index changes. The nature of this 



change is such that the expression , called the " specific 



refractive power/' where n is the refractive index and d the 

 density, is approximately constant for different temperatures. 

 Now, if we take the molecular weight of the body and multiply 

 it by this constant quantity, we obtain a number which is called 

 the molecular refracting-power of the body. Since the mole- 

 cular weight depends on the chemical constitution, it follows 

 that the molecular refracting-power depends on the chemical 

 constitution also. From numerous experiments it has been 

 found that : — 



(a) Isomerides and metamerides have the same molecular 

 refracting-power. 



(b) In a series of compounds which differ by H 2 , the mole- 

 cular refracting-power differs by 2*6. 



(c) In homologous series the molecular refracting-power in- 

 creases 7 *6. 



