328 Dr. J. H. Gladstone. Refraction Equivalents of [Jan. 27, 



lents of the Aromatic Hydrocarbons and their Derivatives ;" together 

 with a discourse at the Royal Institution in March, 1877, on " The 

 Influence of Chemical Constitution on the Refraction of Light." In 

 the meantime, observations on many substances have gradually accu- 

 mulated in my note-book. 



Of late, however, the importance of the subject in regard to theories 

 of chemical structure has been recognised by Dr. Thorpe and other 

 chemists in this country, and attention has been recalled to it in 

 Germany by the papers of Briihl, who, following closely in the footsteps 

 of Landolt, has endeavoured to explain the results in the language of 

 modern organic chemistry. 



At this juncture it may be of service to put on record my present 

 views in regard to the refraction equivalents of the four principal 

 constituents of organic bodies — carbon, hydrogen, oxygen, and ni- 

 trogen. The figures in this paper are always reckoned for the line A 

 of the solar spectrum, the refraction equivalent being the specific 



refraction for A multiplied into the atomic weight, or — -". In 



d 



the present stage of the inquiry, though the results are deduced from 

 many observations, I have not thought it desirable to go beyond the 

 first place of decimals. 



Carbon . — Carbon in its compounds has at least three equivalents of 

 refraction, 5 - 0, 6"0 or 6*1, and about 8*8. 



Whether its refraction should be one or other of these appears to 

 depend on the way in which the atoms are combined. 



When a single carbon atom has each of its four units of atomicity 

 satisfied by some other element, it has a value not exceeding 5'0. 

 There are some indications that the value may be slightly less than 

 this. 



When a carbon atom has one of its units of atomicity satisfied by 

 another carbon atom and the remainder by some other element, it has 

 the value of 5'0, the same as in diamond. This is also the case if two 

 of its units of atomicity are satisfied by carbon atoms. The majority 

 of organic compounds of course fall into this category. 



When a carbon atom has three of its units of atomicity satisfied by 

 other carbon atoms, its value is 6'0. The most striking instance is 

 that of benzol, C 6 H 6 (refraction equivalent 43' 7), in which it is diffi- 

 cult to conceive that each carbon atom is not in the condition just de- 

 scribed, and which, reckoning 1*3 for each hydrogen, gives a little less 

 than 6*0 for each carbon. Styrol, C 8 H 8 (57"8), gives a similar value. 



There are other organic compounds in which only some of the 

 atoms of carbon have the higher value. It has been especially the 

 work of Briihl to point this out, and to show that where they occur 

 (as in amylene or the allyl compounds) the carbon atom is in a con- 

 dition similar to those in the phenyl nucleus, that condition in fact 



