THERMOCHEMISTRY , a See also:

• BRANCH (from the Fr. branche, late Lat. branca, an animal's paw)

See also:

• DAVY, SIR HUMPHRY

Thus, in the above example, it is immaterial whether M displaces M" from its salt directly, or whether M first displaces M', which is then used to displace M". This important principle is a direct consequence of the law of the conservation of energy, but was discovered independently by Hess from accurate experiment. Hess employed this principle to determine indirectly the heat of formation of compounds from their elements, when this magnitude, as is generally the See also:

• CASE
• CASE, JOHN (d. 1600)

Hess now observed that in the See also:

• PROCESS

See also:

• BERTHELOT, MARCELLIN PIERRE EUGENE (1827–1907)

Since the intrinsic energies of the two systems under given conditions are invariable, the difference between them is constant, so that the heat evolved when the first system is converted into the second is equal to that absorbed when the second system is re-transformed into the first (cf. Lavoisier and Laplace, ante, § 1). The See also:

• TOTAL

(3) Change of See also:

• VOLUME

Each gramme-See also:

• MOLECULE (from mod. Lat. molecula, the diminutive of moles, a mass)

It is of course in such a case necessary to know the specific heat of the liquid in the calorimeter. Thomsen by direct experiment found that the heat-capacity of a dilute aqueous solution diverged in general less than t per cent. from the heat-capacity of the water contained in it, the divergence being sometimes in one sense, sometimes in the other. He there-fore abstained from determining for each case the specific heats of the solutions he employed, and contented himself with the above approximation. Berthelot, on the other See also:

• HAND
• HAND (a word common to Teutonic languages; cf. Ger. Hand, Goth. handus)
• HAND, FERDINAND GOTTHELF (1786-185r)

In the newer type (which was first proposed by Andrews for the combustion of gases) the chemical action takes place in a completely closed combustion chamber of sufficient strength to resist the pressure generated by the sudden action, which is often of explosive violence. The See also:

• STEEL, FLORA ANNIE (1847– )

K. The centuple calorie is the amount of heat required to raise 1 g. of water from o° C. to too° C., and is approximately equal to too cal. The large calorie is equal to r000 cal. In view of the not very great accuracy of thermochemical measurements, the precise See also:

• DEFINITION (Lat. definitio, from de-finire, to set limits to, describe)

It is of course necessary in accurate work to state the conditions of the reaction. In the above instance the sulphur is supposed to be in the solid rhombic modification, the oxygen and sulphur dioxide being in the gaseous state, and the initial and final systems being at the ordinary temperature. Again, the equation [2N, 01=—18500 cal. indicates that it 28 grammes of See also:

• NITROGEN [symbol N., atomic weight 14.01, 0=16]

To the right-hand member of the equation he then adds the number expressing the thermal effect of the reaction, heat-evolution being as before counted positive, and heat-absorption negative. The See also:

• MASS (O.E. maesse; Fr. messe; Ger. Messe; Ital. messa; from eccl. Lat. missa)
• MASS, IN

The following equations give the result of direct experiment:—C+20 = See also:

• CO2

Since heats of formation afford such convenient data for calculation on the above method, they have been ascertained for as many compounds as possible. Substances with positive heats of formation are termed exothermic; those with negative heats of formation are termed endothermic. The latter, which are not very numerous, give out heat on decomposition into their elements, and are more or less unstable. Amongst endothermic compounds may be noted hydriodic acid, HI, See also:

• ACETYLENE