178 



HEAT. 



Density Of Saturated Vapour. The densities of various saturated 

 vapours, that is, of vapours in equilibrium with their liquids, have been 

 studied by Hering. The principle of his method will be understood 

 from the following imaginary experiment. Let A (Fig. 103) be a 

 graduated tube closed at the top, and connected by a flexible tube with 

 B open to the air at the top. Let A be entirely filled with mercury, 

 which passes round the bend into B. Let a small weighed quantity of 

 the substance be floated round through the mercury into A. By depres- 

 sing B sufficiently, the pressure in A may be so reduced that the liquid 

 begins to evaporate. At first the vapour in A is saturated, and exerts 

 the vapour-pressure so that, as B is lowered still farther, the two mercury 

 levels continue to fall equally. The vapour in A merely increases in 

 quantity at the constant vapour-pressure. But when all the liquid has 

 just disappeared, the volume occupied by the vapour in 

 A is that of a known weight of vapour saturated at the 

 observed temperature. The point of disappearance may 

 be determined from the fact that, as soon as it is passed, 

 the level in B begins to fall more quickly than in A. 

 In practice it was found better to work in the opposite 

 direction, since, through adhesion to the glass, evapora- 

 tion was not complete till after the saturation-pressure 

 was passed. The point at which condensation begins 

 was therefore found. Hering used a much better but 

 more complicated manometer. He found that for dif- 

 ferent substances the density might be nearly repre- 

 sented by 



Density of saturated-vapour = Density as compared 

 with that of hydrogen at the same temperature 

 and pressure, calculated from the molecular 



weight x y temperature measured from - 273 x 



constant, 



the constant being the same for all the substances 

 experimented on.* 

 FIG. 103. Measurements of Latent Heat. We have already 



seen that, when a liquid boils, its temperature normally 

 remains constant, the heat supplied going to change the liquid into gas 

 without rise of temperature, and we have called this " latent heat." But 

 the term has a precise signification which may be expressed as follows : 



The latent heat of a vapour at a given temperature is the quantity of 

 heat required to convert 1 gramme of liquid into vapour at its maximum 

 pressure at that same temperature. Thus the latent heat of steam at 

 100 is the heat required to convert 1 gm. of water at 100 into 1 gin. 

 of steam at 100, and 760 mm. pressure. 



The general principle of the method of measurement of latent heat 

 will be best understood by considering a rough experiment with steam : 



A flask containing water, with a tube passing through the cork and 

 nearly reaching to the bottom, is inverted, as in Fig. 104, and the water 



* A series of researches has been made by Fairbairn and Tate on the density of 

 saturated steam. For an account of their method the reader is referred to Baynes's 

 Thermodynamics, p. 181. 



