HEAT.— Heats

anhydride, and the heat of combustion obtained willbe less than that due to the chemical action. To findthe heat developed in such a combination it wouldbe necessary to estimate the amount of heat renderedsensible, and to know the total amount of molecularenergy in the carbon, in the oxygen, and in the car-bonic anhydride formed, as the heat produced bychemical action is equal to the molecular energy ofthe compound minus the molecular energy of bothconstituents (B. Stewart). But there are no certainmeans of directly estimating these energies, and weare therefore compelled to combine a certain quantityof carbon in a definite molecular condition and at adefinite temperature, with oxygen, also at a definitetemperature and pressure, and to ascertain by calor-imetry the amount of heat evolved. We shall hererefer to two other cases. In the formation of waterby'the combustion of hydrogen—1st., the two gasesunite; 2nd., they condense in the relation of 3 to 2 toconstitute vapour of water; 3rd., this vapour passesinto the liquid condition. These three actions developin turn a certain amount of heat, and the heatmeasured is the sum. Again, when sulphur combineswith carbon we may say—1st., that the two solidspass into the gaseous state, which causes an absorp-tion of heat; 2nd., that they combine, which causesan evolution of heat; 3rd., that the sulphide formedreturns to the solid condition, thus liberating itslatent heat. The thermal units obtained are conse-quently only the difference between these inverseactions.

The most natural hypothesis, in explanation ofthese phenomena, is that the total heat disengagedor absorbed during a combination is equal to thealgebraic sum of the heats that would be individuallydisengaged or absorbed by each of the changes whichoccur during the process, if they were successive andindependent. And with respect to the same thermalphenomena of chemical action, the most generalresult may be held to be that the evolution orabsorption of heat in any reaction depends upon therelative stability of the system before and after thatreaction (Odling). The passage from a less stableto a more stable arrangement of atoms is alwaysattended with evolution of heat, whether the changebe one of combination, as of phosphorus with oxygen,or of decomposition, as in the explosion of nitro-glycerin or gun-cotton. This change from a state ofunstable chemical equilibrium into a stable chemicalequilibrium may be compared to the falling of a weightto the ground when its support is removed. As theone is attended with a certain expenditure of actualmotion, so is the other with an expenditure of heat,which is interstitial motion. In attaining chemicalequilibrium the elements have formed simple com-binations. Just as in raising the weight again acertain amount of force must be expended, so alsoin separating the elements thus combined, we mustexert a direct chemical action, and expend a certainamount of heat or its equivalent of some other force.In their free state the elements possess a potentialenergy which they lose in combining, and whichmust be restored to them when they are separatedvol. n.

of Combination. 101

again. For every 2 grams, of hydrogen that unitewith 16 grams, of oxygen to form water, there areevolved about 68 thermal units, this unit being thequantity of heat required to raise 1 kilo, of water1° C. To separate the hydrogen again the samenumber of units must be restored. Zinc whenburnt in the air or in oxygen evolves 86 units ofheat for every equivalent (65) in grams, which com-bines with an equivalent in grams. (16) of oxygen.When dissolved in acidulated water it is oxidized atthe expense of the hydrogen, which escapes as gas,the separation of which involves the expenditure of68 thermal units. The oxidation of 65 grams, ofzinc in water is 86 — 68 = 18, and is thereforeonly attended with an evolution of 18 thermal units,the remaining 68 constituting the potential energyof the escaping hydrogen.

Heat Evolved during Decomposition , or ChemicalCombinations Attended with Absorption of Ileat .—Inthe process of oxidation heat is generally evolved;but it is probable that oxygenated combinations,obtained by indirect methods, may be accompaniedby an absorption of heat. Tiienard remarked thatthe decomposition of hydric peroxide is attendedwith the evolution of heat. Favre and Silbermannstate that in this case 1 gram, of oxygen in being setfree produces 1303 thermal units. In the combina-tion of the additional atom of oxygen with themolecule of water it is therefore probable that heatis absorbed, 1 gram, of oxygen absorbing 1303 units.The protoxide of nitrogen also, on being decomposed,disengages heat. Dulong found that when carbon,carbonic oxide, or hydrogen is burnt in nitrousoxide, the evolution of heat is greater than whenthe combination occurs in oxygen. Favre andSilbermann calculated the amount of heat thusevolved in the decomposition of nitrous oxide byburning in it a given weight of carbon. The totalheat produced was the sum of that due to the com-bustion of the carbon, and of that which proceededfrom the separation of the constituents of the nitrousoxide. The former being known, the latter couldreadily be calculated. For each gram, of oxygenset free 1090’5 units are given out. Consequently1090’5 units should be absorbed in the combinationof 1 gram, of oxygen with nitrogen.

In the formation of all the nitrogen oxides thereis an absorption of heat, which is greater with thedioxide than the monoxide, and gets gradually lessfor the remaining three. Their formation beingattended by an absorption of heat, their decom-position will necessarily lead to an evolution of heat.These oxides therefore are never formed by directcombination, some external agency being requiredto cause the gases to unite. Any nitrate accordingly,on account of containing an oxycom pound of nitrogen,will evolve heat when it,is decomposed; and thispartly explains the force possessed by gunpowderand other explosives, where part of the energy isdue to the heating of the gases produced. KNO gevolves 92,800 units, and NaN0 3 85,600 units onbeing decomposed.

Evolution of heat also occurs in the decomposi-21