Intelligence and Miscellaneous Articles. 139 



high pressure or a vacuum. Sometimes the tube has been used 

 for liquids which act upon the metal it is made of ; when this has 

 been the case, a rubber tube has been threaded through the metal 

 tubes, so as to form a protective lining. 





ON EVAPORATION AND SOLUTION AS PEOCESSES OF 

 DIFFUSION. BY PROF. J. STEFAN. 



In a paper published in 1873 the author described experiments 

 which he made on evaporation from narrow tubes. Those obser- 

 vations led to the law that the velocity of evaporation is inversely 

 proportional to the distance of the surface from the open end of 

 the tube. The application of the theory of the diffusion of gases 

 to this process led to the same law, and at the same time furnished 

 a complete determination of the velocity of evaporation, which 

 renders it possible to calculate the coefficient of diffusion of vapours. 

 These experiments have been extended by Winkelmann to several 

 series of liquids, and have been used to determine the coefficients 

 of diffusion of their vapours. 



Similar experiments to those on evaporation may be made on the 

 solution of solids in liquids. A rectangular prism of rock-salt 

 was made ; its height was 30 millim., and the two other dimensions 

 were 7 and 9 millim. Glass plates were cemented by Canada 

 balsam to the bottom and sides, so that only the top surface of the 

 prism was free. In ore of the glass plates a scale is etched. If 

 the prism, with its top upwards, is immersed in a large vessel of water, 

 its solution takes place from the top and the process can be 

 observed on the scale. After 1, 4, 9, 16 days the solution had 

 extended to 6*3, 12*6, 18-8, and 25 millim. These depths are as the 

 square roots of the times. Hence for this process the law holds 

 that the velocity of the solution is inversely proportional to the 

 distance of the rock-salt surface from the open end of the prism. 



If such a prism is dipped with the free surface downwards, the 

 solution proceeds with almost uniform velocity. In one hour 17*1 

 and in 1| hours 25"6 millim. were dissolved. A prism of a metre 

 in magnitude requires for its solution from upwards 70 years, and 

 downwards 2| days ; the former times increase with the magnitude 

 in a quadratic, and the latter in simple ratio. 



Experiments of the first kind may be used for investigating the 

 diffusion of salts through their solvents. It is necessary for this 

 to represent the process in a form which can be calculated from 

 the theory of diffusion. This gives a new method for determining 

 the coefficient of diffusion of salts. The method is not restricted 

 to such bodies as can be obtained in large crystals. If a uniform 

 mixture or a magma is formed of the powder and its saturated 

 solution, and if a graduated tube is filled with it, the progress of 

 the solution can be as well observed by it as with a prism of rock- 

 salt. The law in this case is the same as in the former case, 

 though the absolute value of the velocity with which the plane ot 

 separation of the solution and of the magma moves downwards 



