72 THE VOYAGE OF H.M.S. CHALLENGER. 



the two branches of the isothermal join one another by what is practically a part of 

 that asymptote : — thus making the liquid and the vaporous stages continuous with 

 one another by means of a portion very nearly straight and parallel to the pressure 

 axis. Somewhere on this will be found one of the points of inflection of the isothermal, 

 the other being at a somewhat smaller volume, and at a pressure which is moderate for 

 temperatures close to, but under, the " critical point," but commences to increase with 

 immense rapidity as the temperature of the isothermal is lowered. All the isothermals 

 will now present the same general features, dependent on the existence of two 

 asymptotes and two points of inflection, whether they be above or below the critical 

 point ; but their form will be modified in different senses above and below it. The 

 portion of the curve which is convex upwards will be nearly horizontal at the critical 

 point, and will become steeper both above and below it ; but pressure and volume 

 will nowhere increase together. This suggestion, of course, like that in the second 

 extract above, is essentially confined to the case of a fluid mass which is supposed to 

 have no boundaries ; for their introduction at once raises the complex difficulties 

 connected with the surface-skin. Thus it will be seen that the conviction that water 

 has large molecular pressure has led me back to what is very nearly the first of the two 

 hypotheses I proposed. 



A practical application of some of the principles just discussed is described in the 

 following little paper :— 



ON AN APPLICATION OF THE ATHOMETER. 1 



" The Atmometer is merely a hollow ball of unglazed clay, to which a glass tube is 

 luted. The whole is filled with boiled water, and inverted so that the open end of the 

 tube stands in a dish of mercury. The water evaporates from the outer surface of the 

 clay (at a rate depending partly on the temperature, partly on the dryness of the air), 

 and in consecjuence the mercury rises in the tube. In recent experiments this rise of 

 mercury ha.s been carried to nearly 25 inches during dry weather. But it can be carried 

 much farther by artificially drying the air round the bulb. The curvature of the capil- 

 lary surfaces in the pores of the clay, which supports such a column of mercury, must 

 be somewhere about 14,000 (the unit being an inch). These surfaces are therefore, 

 according to the curious result of Sir'W. Thomson (Proc. Eoy. Soc. Edin., p. 63, 1870), 

 specially fitted to absorb moisture. And I found, by inverting over the bulb of the 

 instrument a large beaker lined with moist filter-paper, that the arrangement can be 

 made extremely sensitive. The mercury surface is seen to become flattened the 

 moment the beaker is applied, and a few minutes suffice to give a large descent, pro- 

 vided the section of the tube be small, compared with the surface of the ball. 



1 Proc. Roy. Soc. Edin., vol. xiii. pp. 116, 117, 1835 



