Specific Heat of Saturated Vapours

We see at once that the specific heat of a saturated vapour may be negative. The second term of (6) is always 0, since 

If we compress the vapour and wish it to remain saturated, we must abstract heat. If we do not abstract heat, but compress the vapour adiabatically, part of the vapour will condense. On the other hand, if we expand the vapour adiabatically it will become unsaturated. 

If the vapour is to remain saturated during compression, we must withdraw heat from it. An adiabatic compression will, therefore, cause the vapour to become unsaturated or superheated, an adiabatic expansion will produce partial condensation. 

Cp = 0. The vapour will remain saturated on adiabatic compression or expansion. 

We can simphfy equations (6) and (7) very materially, by considering evaporation at so low a temperature that we may assume the vahdity of the simple gas laws at least approxi- 

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Clausius (1850), in considering Regnault s data for the latent heat of steam, introduced a new specific heat, applicable to either phase of a saturated complex of two phases, viz., the amount of heat absorbed in raising the temperature of unit mass of a saturated phase by 1°, the pressure being at the same time varied so as to preserve the substance in a saturated state. In the case of a vapour, this is called the specific heat of saturated vapour (a). 

Although the theory of the specific heat of saturated vapour has been developed in several ways, few direct experiments on the lines of those of... 

The Change of the Specific Heat of Saturated Vapour with Temperature... 

Let cf = specific heat of vapour at constant pressure, cr" = specific beat of saturated vapour,... 

Van Wijk and Seeder s viscosity equation, 91 vapour, density of saturated, 324 specific heat of saturated, 336, 346-7, 359 vapour pressure 226 alignment chart, 271 of aliphatic esters, 286 of alkali halides, 237,243 of benzene, 267 boiling-point method for, 235 in capillary tubes, 367 of carbon, 246 centri fugal force, effect of, on 292 constant, 335, 341 over curved surface, 366 determination of, 227-47 dew-point method, 241 of dibasic acids, 243 dynamical method, 235 effusion method, 241. electrification, effect of on, 238, 375 of elements 257 of esters, 250 f., 286 of fusible metal, 230 in... 

From equation (9) it is easy to calculate that the specific heat of saturated water vapour, for example, is negative at the normal boiling point. Thus for 1 gr. of water vapour at 100° C., we have... 

Van Wijk and Seeder s viscosity equation, 91 vapour, density of saturated, 324 specific heat of saturated, 336, 346-7, 359 vapour pressure 226 alignment chart, 271 ... 

These equations are due to Clausius, and give the relation between the specific heats of liquid and vapour in the saturated complex and the ordinary specific heats at constant pressure. 

The adiabatic compression of saturated vapours was considered by Bruhat, who also calculated the angle between the liquid and vapour phase isochores in the entropy-temperature diagram. Amagat investigated the discontinuity in specific heats where an isothermal cuts the saturation curve. Hausen, from a complicated formula for the specific heat of steam involving two Einstein terms ( 2.IX N), calculated the heat content and entropy of steam. Leduc found the value of n for dry steam in Rankine s equation for adiabatic... 

The values of cr" (the sp. ht. of saturated vapour) were calculated it is negative. The specific heats (joules/I° g.) of superheated ammonia vapour were... 

Hagen-Poiseuille law, 72 Hagenbach coefficient, 74 correction, 73, 75 hanging drop, 183, 189 level viscometer, 80 Hare s apparatus, 12 Harkins s equation, 155 heat capacity of electrolyte solution, 225 content of electrolyte and non-electrolyte solutions, 225-6 content of vapour, 348 Heilborn s specific heat formula, 218 Henning s latent heat formula, 307 Herwig s method for density of saturated vapour, 325... 

On the other hand in the case of steam, alcohol, carbon bisulphide, as well as in the case of fluids commonly used in refrigerating machines, etc, carbon dioxide and ammonia, the behaviour is different In these latter cases the specific heat of the saturated vapour remains neg ative throughout the whole range of temperature up to the critical point, at which point its negative value becomes infinite... 

For the sake of simplification we again consider temperatures so low that we may regard the specific heat of the condensate as negligibly small. This does not entail any limitation, for the specific heat may be readily taken into account if necessary. We assume, further, that the volume of the saturated vapour is large compared with that of the condensate, an assumption which is of course perfectly justified at low temperatures. Then, according to the Second Law, we have for the vapour pressure the formula which has already been often employed,... 

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