The third general method of calorimetry, that based on the transformation of some other kind of energy into the form of heat, rests on the general principle of the conservation of energy, and on the experimental fact that all other forms of energy are readily and completely convertible into the form of heat.
The equation of energy is dQ=dE+pdv, (17) expressing that the total energy dQ is used partly in increasing the internal energy of the gas, and partly in expanding the gas against the pressure p. If we take p = RNT/v from equation (14) and substitute for E from equation (16), this last equation becomes dQ 2 (n +3)RNdT +RNTdv (18) which may be taken as the general equation of calorimetry, for a gas which accurately obeys equation (14).
These discrepancies might, no doubt, be partly explained by differences in the units employed, which are somewhat uncertain, as the specific heat of water changes rapidly in the neighbourhood of o° C; but making all due allowance for this, it remains evident that the method of ice-calorimetry, in spite of its theoretical simplicity, presents grave difficulties in its practical application.
But, Quite Apart From This, Electrical Methods Possess The Greatest Value For Calorimetry, On Account Of The Facility And Accuracy Of Regulating And Measuring The Quantity Of Heat Supplied By An Electric Current.
Assoc. Reports, 1897 And 1899) Adopted An Entirely Different Method Of Calorimetry, As Well As A Different Method Of Electrical Measurement.
These subjects are discussed in the articles Density; Thermometry; Calorimetry; Diffusion; Conduction Of Heat; and Condensation Of Gases.
The unit of heat assumed in the table is the calorie at 20° C., which is taken as equal to 4.180 joules, as explained in the article Calorimetry.
(For the exact relation between these heat-units, see Calorimetry.) For ordinary thermochemical work we may adopt the relation 1 cal.
Specific Heat and Composition.-The nature and experimental determination of specific heats are discussed in the article Calorimetry; here will be discussed the relations existing between the heat capacities of elements and compounds.