1201 



111 his papers in the Reciieil he described the refracloirieter con- 

 structed by hini with constant deviation of 40°, which is obtained 

 by rotation of the prism round a vertical axis. This apparatus has 

 further an appliance, by means of which measui-ements of the 

 refraction up to a teinperalure of about J 50° can take place with 

 ease foi' which reason it is to be preferred for organic-chemical 

 researches to all other refractometers. Also his pycnometers are 

 described there. 



It is further demonstrated in these papers that the refractometric 

 value of the group CH, is constant for the most divergent homologous 

 series, if only the first three teims of (hese series are left out of 

 consideration, for which this value is either greater or smaller. 



Besides they contain the derivation of Eykman's formula for the 

 molecular refraction. Hitherto the formula of Gladstone and Dalk : 



?2— 1 



— --. P= const, was generally used for this, in which n represents 

 a 



the index of refraction, (/ the specific weight of the liquid substance, 



and P the molecular weight. After the formula .— had been 



n'+2 (/ 



derived by Lorkntz by a theoretical way, this formula got to be 



almost exclusively used. The formulae of Gladstone and Dale and 



of LoRENTZ do not present a constant value, however, for large 



ranges of temperature (e.g. of 100°); but those of the former descend, 



while the theoretical formula gives ascending values. 



Taking into account that G. & D's formula may also be written: 



n'— 1 P 



.--, and that accordingly the difi^erence between the two for- 



724-1 d ^-^ 



mulae refers only to the denominator, Eykman tried by an empirical 



way to find a formula that also has constant values for large ranges 



71'— 1 P 



of temperatures, and he found it in the expression .— . This 



^ ^ n+0.4 d 



rendered it, therefore, possible, to directly compare measurements 

 which have been made at very divergent temperatures. 



The papers in the ('hemisch Weekblad treat two problems of 

 great importance for organic refractometry, viz. : the cyclic com- 

 pounds and unsaturate substances. As far as the former is concerned, 

 he comes to the result that the numbei- of C-atoms in the nucleus has 

 a considerable influence on the refraction, which also extends over 

 tlie refractometric values which CHj-groups have in the side chain. 



In reference to the unsaturate compounds he proves by means 

 of an exceedingly copious material, that there can be no question 

 of a constant increment for the double binding, which Brühl intro- 



