176 OUTLINES OF PHYSICAL CHEMISTRY
value deduced from the beat of combustion of benzene is equal to 786-84 — 799*85 = — 12-51 Cal. under constant pressure, which is equivalent to — 13*67 Cal. at constant volume. This number is not in agreement with either of those furnished by theory. It corresponds, however,
The road once opened, it has been possible for Thomsen to go much further and to determine the thermal effect corresponding to other interatomic relationships. He has studied, for example, the successive substitution of four atoms of chlorine for the four atoms of hydrogen in methane, and he has found that these four successive operations do not evolve the same quantity of heat. He has further found that the fixation of a hydroxyl group (OH) on to a carbon atom is accompanied by an evolution of heat which varies according as the compound formed is an alcohol or an acid.
The thermal effect of the fixation of an atom or of a radical on to carbon depends, therefore, to a certain degree on the nature of the union of the carbon before this fixation.
The facts and the conclusions which we have just arrived at evidently appear to contradict the starting point of Thomsen' s system. For Thomserts fundamental hypothesis was that, apart from the heat necessary for the isolation of the carbon atom, the reactions [C, 0] and [CO, 0] should produce the same thermal effect.
The Law of Maximum Work
Mechanics teaches us that a system of material points is in equilibrium when the sum of the forces acting on the system has produced the greatest possible quantity of work. A very general rule, which has for a long time been raised to the height of a principle, determines in an
1 The heat of combustion of benzene (in the state of vapour at 18°) is, according to Thomsen, 799*35 Cal.
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