536 



EEPOET — 1880. 



or, to a considerable extent, by chemical combination. It is the refractive index — 1 

 divided by the density. It was originally reckoned, both for the theoretical limit of 

 the spectrum according to Cauchy's formula, and for Frauuhofer's lines B, F, & H. 

 But in all subsequent work, the author has calculated the specific refractive energy 



for the line A, as least affected by dispersion ( ^ — -H- J . For purposes of cal- 

 culation among compound bodies, it is more convenient to adopt what Landolt 

 terms the refraction-equivalent ; that is, the specific refractive energy multiplied by 



the atomic weight 



(^'^> 



Uncombined carbon as found in diamond has a refraction-equivalent varying 

 from 4-85 to 5-18 ; the mean may be taken at 5-0. It has the same value in the 

 large majority of its compoimds, such as bisulphide of carbon, cyanogen, sugar, 

 tartaric acid, alcohol, and the whole of the ordinary bodies of the fatty acid series. 

 It was very early observed, however, that there were exceptions, and it is now 

 kno'WTi that the whole of the bodies belonging to the aromatic series, the terpenes, 

 the pyridine series of bases, cinnamyl compounds, and hydrocarbons which are 

 peculiarly rich in carbon, such as naphthalene, anthracene, &c., give an excessive 

 refraction. This peculiarity, so far as the aromatic bodies and naphthalene are 

 concerned, was sought to be explained in a lecture at the Royal Institution, in 

 March 1877, by the fact that the usual atomicity of the carbon is not satisfied, as 

 illustrated hj the graphic formulae usually employed for this class of bodies. 



Briihl has lately published a series of papers in which, by careful experiments, 

 he has confirmed and extended previous observations, and he endeavours to prove 

 that wherever there is a double carbon atom with bonds latent, the refraction-equi- 

 ■valent is raised by about 2-0. This view answers satisfactorily for the great aro- 

 matic group, for the allyl compounds, for picoline and its congeners, and for 

 amylene, the refraction-equivalent of which is 1-95 above the normal, although 

 the halogen compounds of ethylene, propylene, and amylene are normal. This 

 theory, however, does not seem equally adequate to account for certain other phe- 

 nomena. 1st. The e.ssential oils which belong to the C,oHjg or the Cj^H.^, group, 

 have a refraction which is neither 2 nor 4 above the normal, but somewhere be- 

 tween these numbers 

 oil of cassia, have 



refraction of 85-0, which is 13'4 above the calculated amount, while its isomer, 

 phenyl-ethyl acetate, has only the excess of 6-6 which is usual in phenj-1 compounds. 

 •3rd. The hydrocarbons, which have a greater number of atoms of carbon than of 

 hydrogen, increase in refraction, with the excess of carbon, at a rate which is far 

 more rapid than the theory will admit of, as will be seen from the subjoined table, 

 in which the last column represents the excess of the refraction-equivalent over 

 that calculated from carbon = 5 and hydrogen = 1-3, 



2nd. The cinnamj'l compounds, such as the well-known 

 an abnormal refraction; cinnamene acetate, 0]qH,jO, has a 



It is a remarkable fact that, whereas the value of the carbon increases rapidly 

 as the proportion of hydrogen diminishes, its value reverts to the normal 6-0 in 

 diamond where there is no hydrogen at all. 



The author expressed his belief that the specific refractive energy of a carbon 

 compound is a property which must be taken into account in determining its consti- 

 tution ; and he hoped that some of those chemists who have paid particular attention 

 to the theory of organic chemistry, would take up this line of investigation. 



