— 160 — 



cyclic double linking corresponds with a clearly perceptible exaltation in 

 the power of dispersion. The latter, however, is small and almost lies • 

 within the limits of error. 



It is therefore possible to distinguish with clearness by the spectro- 

 chemical method between alkylq/dohexenes and alkylidene q/cZohexanes. 



In chemical literature a whole series of hydrocarbons is erroneously 

 described as alkylidene cyclohQxane derivatives, which in reality consists 

 of endocyclically unsaturated compounds. Among these bodies, for example, 

 are several hydrocarbons prepared by Sabatier and Mailhe '). 



Finally, Auwers and Ellinger have investigated the q/c?ohexenyl acetic 

 acids and the isomeric cs/cZohexylidene acetic acids, and in particular the 

 esters of these bodies, with the result that they have found that in them 

 there is a marked difference between the compounds with semicyclic double 

 linking and the endocyclic non-saturated isomerides. The latter are in every 

 respect normal, but the former display pronounced exaltation in their 

 coefficient of refraction, and still more pronounced exaltation in their 

 power of dispersion. The reason of this is obvious, inasmuch as the 

 q/cZohexylidene acetic acids not only possess a semicyclic double linking 

 but also a conjugation, and the first effect of the latter is a powerful rise 

 in refraction and dispersion. 



As many of the free acids with semicyclic double linking have high 

 melting points it is advisable to use the esters for optical examinations. 



Hantzsch 2 ) calls attention to another physical property, namely ab- 

 sorption of the ultra-violet rays, which is of great importance in testing 

 the purity of terpenes. Hantzsch has examined «-pinene, d-, l- a ) and 

 i-limonene, dipentene, Z-a-phellandrene, /?-phellandrene, sylvestrene, caryo- 

 phyllene, m- and jp-cymene, d- and Z-camphor, and d- and Z-borneol. All these 

 bodies (of which the experimental material had been placed by us at the 

 disposal of the author), were purified by Hantzsch by fractionation, using 

 a glass-bead column, in a Claisen flask in a current of carbonic acid gas 

 at 10 to 15 mm., from a bath of Wood's metal. The solutions required 

 for the determination of the ultra-violet absorption were prepared with 

 optically pure ethyl alcohol; in order to obviate oxidation it is necessary 

 to pipette the freshly-prepared distillates in measuring-flasks which have 

 previously been filled with carbonic acid. Hantzsch does not give the 

 constants of the bodies examined by him, which is to be regretted, because 

 they would have made it possible to ascertain in how far the properties 

 of the preparations in question are influenced by impurities. In the exa- 

 mination of terpenes distilled in vacuo it soon became evident that me 

 absorption-method is much more sensitive than the refraction-method, 



1 ) Compt. rend. 139 (1904), 343. 



2 ) Berl. Berichte 45 (1912), 553. 



8 ) In his paper, Hantzsch erroneously speaks of Z-limonene as being prepared from 

 orange oil. As a matter of fact, this terpene occurs in pine-needle oil. 



