Thermo Barometer
Collier's New Encyclopedia (1921)
an instru-vance in elucidating the theories underlying thermo-chemical phenomena. Three of the fundamental principles which he laid down are: (1) The thermal change due to a chemical reaction depends (providing no external work is done) only on the condition of the system at the beginning and end of the reaction, and not on the intermediate conditions. (2) The heat evolved in a chemical reaction is a measure of the corresponding physical and chemical work. (3) Every chemical change which occurs without the addition of external en- THERMO CHEMISTRY. All chemi-ergy tends to produce that substance or system of substances the formation of which is accompanied by the evolution of the maximum amount of heat. This generalization is found to be not true in every case, but it applies to so many instances that it is generally held to indicate some natural law which is not, at present, fully understood. The Heat of Formation of a substance is the amount of heat liberated or absorbed when the substance is produced by a direct combination of the elements of which it is composed. It is customary to determine the heat of formation on gram-molecular weights; so that, quantitatively, the heat of formation of any substance is expressed as the amount of heat liberated or absorbed when a grammolecular weight of the substance is formed by combination of its elements. In those cases where compounds cannot be formed from direct combination of elements, it is necessary to determine the heat of formation indirectly. This is commonly done by first burning the elements in oxygen, then burning the compound in oxygen, and determining in each case the heat liberated. The products of combustion will, in each case, be the same, so that the difference in the heat liberated in the two cases will give the heat of formation of the compound.
What Has Changed in the Last Century
The text printed here under "Thermo Barometer" is actually Collier's 1921 article on thermochemistry, and its most dated passage is the third principle: that a spontaneous chemical change tends to produce whatever substances release the most heat. Collier's admitted this Thomsen–Berthelot rule "is found to be not true in every case" but held that it pointed to "some natural law which is not, at present, fully understood." That law had in fact been published by J. Willard Gibbs in 1876: it is not heat released (enthalpy) but free energy, G = H − TS, that decides direction, which is why endothermic reactions can run spontaneously when entropy rises. The first principle, Hess's law of 1840, remains exact, being a consequence of conservation of energy; the instrument itself, a thermobarometer or hypsometer measuring altitude from the boiling point of water, has been displaced by aneroid and GPS altimetry.
Written for this edition, 2026.