THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[SIXTH SERIES.] 



JAJSfUAEY 1905. 



I. On thp, Boiling-Points of Homologous Compounds. 

 By Sydney Young, D.Sc, F.R.S., Trinity College, Dublin*. 



IT was in 1812 that Kopp first made the statement that a 

 constant difference in chemical composition is accom- 

 panied by a constant difference in boiling-point. He at first 

 adopted the value 18° as the rise due to the replacement of 

 the ethvl by the methyl group in organic compounds, but 

 two rears later, after making a larger number of comparisons, 

 he concluded that 1 ( J° was the true value, and he suggested 

 that this is the constant difference in boiling-point for an 

 addition of CH 2 in any homologous series. 



As a result of his own determinations and those of other 

 experimenters, carried out during the following twenty-three 

 vears, Kopp was finally induced to give up some of his 

 earlier conclusions. He admitted that isomeric compounds 

 may have different boiling-points, and that the rise due to an 

 addition of CH 2 is not invariably 19°, but he still maintained 

 that in any given homologous series the rise is constant : 

 thus, for homologues of toluene he adopted the value 20 o, 5, for 

 those of xylene 1 8°*5, and for those of trimethyl benzene 16°'5. 



As a pioneer, Kopp had very great difficulties to contend 

 with when he began his researches; data were scanty and 

 far from accurate, and the substances which could be most 

 easily obtained and, it was thought, readily purified were, 

 unfortunately, those which were the least likely to lead to 

 normal generalizations. Water, the alcohols, and the organic 

 acids all contain a hydroxyl group; and we now know that 

 the physical properties of these substances are abnormal in 

 nearly all respects, owing, probably, to the fact that their 



* Communicated by the Author, beinp' ;i development of the first 

 part of the Presidential Address to Section 1> of the British Association. 



Phil. Mag. 8. 6. Vol. 'J. No. 40. Jan. 1905. B 



