468 Dr. T. Carnelley on the 



it follows that the molecular refractive energy of a compound 



is equal to the sum of the atomic refractive energies of its 



components ; thus : — 



Molecular refractive energy. 



, -A. , 



Found 

 Calculated. = /^zl Xm \ 



C 4 H 10 O=(4x5) + (10xl-3)+ 3 =36-0 36-3 



C 2 H 4 2 = (2x5) + (4 xl-3) + (2x3) = 21-2 21-2 



Similar relations also hold good in regard to compounds of 

 the aromatic series ; but their molecular refractive .energies 

 are higher than those calculated from the values for C, H, 

 and given above. Landolt (Chem. Centr. 1S72, p. 705) 

 has shown, however, that a constant refraction-equivalent is 

 obtained for the benzene nucleus in its various derivatives 

 when the refraction-equivalents taken for the C, H, and of 

 the side chains are those given above in the case of the fatty 

 series. 



Gladstone (Phil. Trans. 1870, p. 9) and Haagen (Pogg. 

 Ann. cxxxi. p. 117) have also, by a similar method, calculated 

 the refraction-equivalents of a large number of elements, and 

 have shown that, as a general rule, each element retains its 

 own refractive power with whatever other elements it may be 

 combined, and also that the specific refractive energies are, 

 for the most part, greater as their combining-powers are less. 

 Lothar Meyer (Modernen Theorien der Chemie) has likewise 

 pointed out that the atomic refractive energies, or refraction- 

 equivalents, of the elements are a periodic function of their 

 atomic weights. 



Photographic Transparency. — As regards the photographic 

 transparency or diactinic power of inorganic substances, 

 Miller (Phil. Trans. 1863, p. 1; Chem. Soc. Journ. xviii. 

 p. 59) found that fluorides are chemically the most transpa- 

 rent, after which come the chlorides of the alkalies and alka- 

 line earths, then the bromides, and finally the iodides. Here 

 the diactinic power diminishes with the atomic weight of the 

 halogen. The nitrates, as a class, possess a remarkably high 

 absorptive power for chemical rays, whereas the reverse is the 

 case as regards the chlorates. 



AbsorpAion of the Ultra-violet Pays of the Spectrum by Orga- 

 nic Substances. — This subject has been investigated by Hartley 

 and Huntington (Nature, xix. p. 259), who find : — (1) The 

 normal alcohols C n H 2M +i OH are remarkable for transparency 

 to the ultra-violet rays of the spectrum. (2) The normal fatty 

 acids show a greater absorption of the more refrangible rays 



