1887.] 



Dispersion Equivalents. 



405 



Carbon. — Schrauf's* observations upon diamond give 0"058 for the 

 dispersion equivalent of the same range. 



Iodine, in the state of vapour, or dissolved in bisulphide of carbon, 

 gives a spectrum in which the order of the colours is abnormal. 



Far more important results have been obtained from organic sab- 

 stances, by following a method similar to that which Landolt adopted 

 in his determination of the refraction equivalents of carbon, hydrogen, 

 and oxygen. The materials for such an enquiry are very abundant. 

 They consist of the observations published by Mr. Dale and myself in 

 1863, and my more recent determinations published and unpublished, 

 the very valuable lists of Landolt and Briihl, numerous observations 

 by Kanonnikoff, ISTasini, and others. The Continental observers have 

 usually adopted the lines <x, /3, and 7 of the hydrogen spectrum. 



On comparing the refraction equivalents of organic liquids of the 

 fatty acid series which differ from one another by CH 2 , or multiples 

 of it, my best determinations lie between 0"33 and 0'36, averaging 

 about 0*35 for each CH 2 . On treating in a similar manner fifteen 

 series of such bodies in Bruin's tables, some of which contain many 

 terms, the dispersion equivalent for 7 — a, works out very uniformly at 

 an average of 0'215. This answers to 0'342 for H — A. Armstrong's 

 cymhydrene, which is a saturated substance of the formula C 10 H 20 , 

 has a dispersion equivalent of 3"44, giving therefore 0'344 for each 

 CH 2 . Kanonnikoff's determinations of tetraterpene and naphthene, also 

 C 10 H 20 , give similar numbers. It may therefore be assumed that the 

 value of CH 2 in saturated organic compounds lies between 0'34 and 

 0'35, answering to the well known 7 '6 as the refraction equivalent of 

 the same combination. When, however, we examine unsaturated 

 compounds in a similar manner, we find that the value rises to at 

 least 0*40. 



Hydrogen. — While the value of CH 2 may be fairly taken at 0'34, it 

 is more difficult to say what portion of this is due to the carbon, and 

 what to the hydrogen. I have endeavoured to determine it, by 

 deducting n times CH 2 from the paraffines C w H 2>l+2 ; by comparing 

 the monatomic, diatomic, and triatomic alcohols, and by other 

 similar means. The results are somewhat irregular, as might indeed 

 be expected from the smallness of the residual figure, but give a 

 mean of - 04 per each hydrogen. 



Carbon— If the H 2 in CH 2 be taken at 0'08, it follows that the 

 carbon will have a dispersion equivalent of about 0"26. This answers 

 to the refraction equivalent of 5'0. 



It is well known, especially from the researches of Briihl, that in 

 unsaturated organic compounds, there is an increase of refraction, for 



* " TJeber das Dispersionsaquivalent von Diamant." ' Wiedemann, Annalen,' 

 vol. 22, 1884, p. 424. 



