on the Physical Properties of Compounds. 183 



second part of the journey of a stone which had first fallen 

 only one quarter of the way towards the earth and afterwards 

 fell the remaining three quarters. For a stone falling from a 

 given height to the centre of the earth can only produce one 

 given quantity of heat : and if it divides its fall into two parts. 

 then the greater the amount of heat produced at the end of the 

 first portion of its fall the less will be the amount produced in 

 the second part of the fall. 



And so it is in the formation of chemical compounds. Sup- 

 pose we have a compound of carbon and hydrogen ; this, on 

 complete combustion, gives out a fixed and definite quantity of 

 heat, say 3a\ If now, instead of being completely burnt to car- 

 bonic acid and water, the process stops at an intervening 

 point with the formation of two isomeric compounds A and B, 

 and if * units have been evolved in the formation of A and 

 2x units in the formation of B, then the heat evolved by the 

 further complete combustion of A and B will be 2x and x 

 respectively; i. e. of two isomeric compounds, the one which 

 has the greatest heat of formation will have the least heat of 

 combustion, and vice vend. TVe have also seen, that the greater 

 the heat evolved in the formation of a compound from its ele- 

 ments the greater is the attraction between its atoms, and 

 therefore the higher the melting-point. So that, of two iso- 

 meric compounds, that which has the highest melting-point 

 will also have the greatest heat of formation and least heat of 

 combustion. 



Xow though isomeric compounds, as a rule, have nearly the 

 same heat of formation, and therefore of combustion, yet they 

 are not exactly the same in this respect. This being so, what 

 is it that causes the difference, small though it may be ? The 

 cause can be nothing but the difference in the arrangement 

 of the atoms in the two compounds. And from a comparison 

 of all available data, which in this respect are very meagre, 

 I venture to conclude 



(1) TJiat the stability, and therefore the heat of formation, of 

 symmetrical compounds {and therefore those with highest melting- 

 points) is greater than that of asymmetrical compounds isomeric 



with them. 



If this be true, it will follow 



(2) That the heats of combustion of the former compounds 

 are less than those of the latter. 



The following table contains all the examples to which the 

 above hypothesis can at present be applied: — 



