196 Regularity in the Properties of analogous Compourids. 



which the comparison between the specific volumes should 

 properly be made, have always been neglected. Without 

 doubt such a comparison is, strictly speaking, only then ad- 

 missible when the temperatures are equidistant from the 

 boiling points ; but all observations have been made at mean 

 temperature, and- this accounts for the differences between the 

 results ot calculation and those of observation. On this ac- 

 count all the numbers in the above laws must be regarded as 

 being merely approximative. 



The less the boiling points of the corresponding compounds 

 differ tVom each other, the greater will be the agreement of 

 the calculated results with those of observation. Since the 

 corresponding compounds of tethyl and methyl differ only 

 by 18^ C. in their boiling points, the neglecting the circum- 

 stance of at what temperature the specific weights should be 

 compared, has very slight influence in tiie consideration of 

 these compounds; but where it is a question of replacement 

 of hydrogen by chlorine it is totally different, as the boiling 

 point rises rapidly with the increase of chlorine. 



Unfortunately, the observations on the boiling points of 

 substitution compounds are far more contradictory than those 

 on their specific weights, and the uncertainty is too great to 

 allow us at present to state with accuracy how many degrees 

 the boiling point rises when x atoms of chlorine take the 

 place of .r atoms of hydrogen. 



The best observations seem however to indicate, that for 

 substitution compounds, which can be compared with regard 

 to their condensation or specific weight in the gaseous form, 

 the boiling point is raised x. 12^ C. by the substitution of a; 

 atoms of chlorine for x atoms of hydrogen. Let D be the 

 number of degrees the boiling point is raised by the substitu- 

 tion of one atom of chlorine tor one of hydrogen, we have 

 from the best observations, — 



Hydrated acetic acid and chloro-acetic acid, 



C* W iY 120 Liebig n j3 _ ^50. D _ J20.5 



C4 H2 O^Cl^ 195 Dumas j-^ U - 73 , U - 1^ ^. 



Aldehyd and chloral, 



C^ H« 02 21-8 Liebig 1 g j3 _ 72.0. D - l2°-0 



Several other observations, which however I do not regard 

 as perfectly admissible, and do not consider as fit for the esta- 

 blishment of the law, agree in this respect. If we calculate 

 for several substitution compounds the boiling points of all 

 the other compounds from tliat which has the lowest boiling 



