Influence of Atomic Weight. 467 



constant number. (3) The latent heats of all liquids are 

 simple multiples of the specific heats. 



The following are some of the more important relations 

 which have been traced between the composition of substances 

 and their action, or otherwise, on light. 



Optical Rotation. — According to Pasteur (Reclierches, p. 27), 

 molecules are divisible into two classes : — (1) Those in which 

 the atoms are symmetrically arranged : (2) those in which 

 the arrangement is asymmetrical. The property of optical 

 activity belongs to the latter class only ; and Le Bel has pro- 

 posed the hypothesis that when a C atom is united directly 

 with four different elements or radicals, an asymmetrical form 

 of molecule is produced, which must therefore be optically 

 active. This view is supported hj Van t'Hoff (Bull. Soc. 

 Chim. [2] xxiii. p. 295), who finds, from the comparison of 

 the chemical composition of all known active and many inac- 

 tive bodies : — (1) That every organic combination which 

 rotates the plane of polarized light contains an atom of asym- 

 metrical carbon. The converse of this law, however, does 

 not hold good. This may be attributed either to the presence 

 of two isomerides of opposite rotatory power, or to the fact 

 that asymmetrical carbon is not of itself sufficient to produce 

 optical activity, which may also depend on the nature as well 

 as on the mutual diversity of the atoms attached to the asym- 

 metrical carbon atom. (2) Derivatives of optically active 

 combinations lose their rotatory power when the asymmetry 

 of .their carbon atoms disappears. When the contrary happens, 

 they do not generally lose it. These facts will in the future 

 no doubt render great service in determining the constitution 

 of optically active chemical compounds. 



Specific Refraction. — Gladstone and Dale (Journ. Chem. 

 Soc. [2] viii. pp. 101, 147; x. p. 1), Landolt (Pogg. Ann. 

 cxxiii. p. 603), and others have shown that there is a true con- 

 nexion between chemical composition of compounds and their 



n — 1 

 molecular refractive energy ( = mx — y- , where m = molecular 



weight, n= refractive index, and d= specific gravity). 



The following are the more important points in regard to 

 this subject : — (1) Isomerides and metamerides, if of similar 

 chemical constitution, have the same molecular refractive 

 energy. (2) The molecular refractive energy increases in 

 homologous series by 7*6 for each increase of CH 2 . (3) By 

 the addition of H 2 , the molecular refractive energy increases 

 by 2*6. (4) The molecular refractive energy of the acids 

 C» H 2 » 2 = 7'6tt + 6. (5) The atomic refractive energy of C, 

 H, and are therefore C = 5, H = 1'3, = 3. (6) From thi? 



