172 



HEAT. 



densation nuclei and that condensation occurs under its action even with 

 slight expansions, the fog produced being of a blue colour. Tyndall had 

 previously found this formation of blue fog in many other vapours denser 

 than water, under the action of ultra-violet light. The colour implies 

 that the drops formed are comparable in size with the waves of blue 

 light and so scatter that constituent when white light falls on them. 



Exact Measurements of Vapour-Pressures, Various observers 

 have made researches on the vapour-pressures of liquids at different 



temperatures, the most complete as 

 well as the most important being those 

 in the case of water. 



Two methods have been employed, 

 the "statical" method, which is illus- 

 trated in its simplest form by the 

 experiment described on p. 158, in 

 which the pressure of the vapour 

 depresses the column of mercury in 

 a barometer, and the " dynamical " 

 method, in which advantage is taken 

 of the fact that a liquid normally boils 

 when the pressure of its vapour is 

 equal to the external pressure. 



In the first method, the temperature 

 is varied at will, and the corresponding 

 maximum pressure measured ; and in 

 the second method, the pressure is 

 varied at will, and the corresponding 

 maximum temperature, that is the cor- 

 responding boiling-point, is observed. 



These methods were carried out in 

 Regnault's classical researches on the 

 vapour-pressure of water, which may 

 be taken as typical. He adopted three 

 arrangements, corresponding to three 

 ranges of temperature : (a) From 

 to 50 0. ; (&) below, and up to 0. ; 

 (c) from 50 upwards. 



(a) From to 50. Two barome- 

 ters, AB, A'B', with a common cistern 

 E (Fig. 95) are placed side by side 

 their upper ends being in a vessel containing water which serves as a 

 constant-temperature bath. The front of this vessel is glazed, so that the 

 levels of the mercury in the two tubes may be observed by a cathetometer. 

 D is a spirit-lamp to raise the temperature of the water when required, 

 and F is a stirrer to keep the temperature uniform throughout. A small 

 quantity of water having been introduced into the tube AB, the vapour 

 from it depresses the column, and after correcting for the pressure due to 

 the small excess of water, for the effect of the water on the capillary 

 depression, and for the temperature of the mercury, the difference in 

 level in the two tubes gives the vapour-pressure at the temperature of the 

 bath. Since the vapour-pressure at 50 is only about 92 mm., the batb 



FIG. 95. Vapour- Pressure of 

 Water from to 50. 



