528 Messrs. Owen and Hughes on Condensation Nuclei 

 (30) gives therefore 



dv 2 ~ a(y-by [ ^ N ' 



In the point U just beneath the point G, fx l — ^>0 and 

 the p^ — v curve turns therefore the convex side to the 

 volume-axis. Because at the point H the tangent is parallel 

 to the volume-axis, the p 2 — v curve has a point of flexion 

 between H and U. In the same manner we find that the 

 p2 — v curve HUWVK has also a point of flexion between 

 W and Y, where V is the point just beneath P. 



The curve HUWVK gives therefore together with the 

 empiric isotherm HK a complete image of the relation 

 between the hydrostatic pressures (p 1 and p 2 ) and the 



reciprocal value v = - of the density in a point of the 



capillary layer, whereas the state of every point of the capil- 

 lary is completely described by these three things. 



L. Condensation Nuclei produced by Cooling Gases to Low 

 Temperatures. By Gwilym Owen, B.A., M.Sc, Assistant 

 Lecturer and Demonstrator in Physics in the University of 

 Liverpool, and A. Ll. Hughes, B.Sc* 



WE have recently examined the condition in which air is 

 evolved from charcoal after absorption in a dry and 

 dust-free state at the temperature of liquid air, in order to 

 determine whether it remains dust-free when liberated by 

 warming the charcoal to atmospheric temperature. The char- 

 coal was contained in the bend of a small glass U-tube 

 connected to the cloud chamber of a Wilson's expansion 

 apparatus ; and, after the liquid air used for the cooling was 

 removed and a sufficient time was allowed for the evolved air 

 to regain the temperature of the atmosphere, a sample of the 

 air was admitted through a glass tap into the cloud chamber. 

 It was at once found that the evolved air contained large 

 numbers of nuclei, sufficient, in fact, to produce dense showers ; 

 but when a control experiment was performed without any 

 charcoal in the U-tube, a similar number of nuclei appeared. 

 The experiments therefore were indecisive as to whether the 

 escape of the absorbed air caused disintegration of the char- 

 coal, but they raised the very interesting question as to the 

 cause of the showers obtained in the expansion chamber when 

 air which had merely passed through a process of cooling 



* Communicated hy Prof. L. R. Wilberforce, M.A. 



