CONTRIBUTIONS FROM THE JEFFERSON PHYSICAL 

 LABORATORY, HARVARD UNIVERSITY. 



ON EVAPORATION FROM THE SURFACE OF A 

 SOLID SPHERE. 



PRELIMINARY NOTE. 



By Harry W. Morse. 



Presented by John Trowbridge, February 9, 1910. Received January 3, 1910. 



The micro-balance of Salvioni and Nernst permits of following small 

 changes in weight with considerable accuracy, provided the body under 

 investigation has a mass not greater than a few milligrams. This bal- 

 ance consists merely of a fibre of quartz or glass, firmly held in a nearly 

 horizontal position by being secured at one end, and provided at the 

 other end with some means of attaching the object to be weighed. The 

 weight is then, within quite wide limits of deflection, proportional to 

 the deflection, and the balance is easily calibrated by means of small 

 riders of known weight, Deflections are followed by means of a cathe- 

 tometer or a microscope with micrometer eyepiece. Differences of 0.01 

 millimeter or even less are easily determined, and if the fibre be so 

 chosen that a weight of 1 milligram gives a deflection of about a centi- 

 meter, there is no difficulty in detecting and measuring changes of 

 weight of 0.001 milligram or less. 



With such a balance the change of weight of small spheres of iodine 

 has been followed at approximately constant temperature. Evapora- 

 tion was allowed to go on in a large box with glass sides, and the two 

 side doors of the case were left open before each series of readings to 

 allow free circulation of air. It may therefore be assumed that the 

 partial vapor pressure of iodine in the atmosphere about the evaporat- 

 ing spheres was constant. The temperature was constant within about 

 0.3° during each run. 



After many attempts to obtain definite geometrical form by casting, 

 fairly accurate spheres were made by pouring molten iodine into water. 

 There is no difficulty in obtaining in this way approximately spherical 

 pieces with radii varying from 1 millimeter down to 0.2 millimeter. 



