20 



CLARKE 



Mean, 13946 



Here the mean value is unusually high, on account of the last 

 six compounds. 



The oxy-derivatives of the aliphatic hydrocarbons, then fall 

 into four groups as defined by the thermal effect of their internal 

 oxygen. In the ethers this effect is normal, and c^ can be ig- 

 nored ; in the other groups we have 



— ^j for the alcohols, 



— 4Cj for the. esters, 



— ^Cy for aldehydes, ketones and acids. 



The esters, it should be observed, are intermediate between the 

 alcohols and the acids, and the coefficient of c^ indicates this 

 peculiarity. The different structural relations of the oxygen 

 atoms are also suggested in these values, but not rigorously ; 

 for in a single compound this element may exercise two distinct 

 functions, which the foregoing coefficients do not clearly 

 show. 



Unfortunately we cannot, for these compounds, write com- 

 plete equations of combustion in the sense in which we have 

 written them hitherto. In the typical equation for methane the 

 symbol — r represents the total heat of dissociation, or of for- 

 mation when the plus sign is used. In a similar equation for 

 methyl alcohol it can only stand for partial dissociation, and its 

 meaning is therefore distinctly changed. Hence, in the equa- 

 tions for this class of substances we are unable to directly 

 evaluate the absolute heats of formation. To estimate these we 

 must apply the corrections which are indicated by the different 



