MORSE. — EVAPORATION FROM THE SURFACE OF A SPHERE. 367 



and the curve has been drawn through the points thus determined. 

 The constant was calculated from the mean of all the observations and 

 shows a probable error of a little less than 0.5 per cent. The results 

 of the observations are given as smaller circles. In putting in the re- 

 sults for the smaller spheres or for those in which a full run down to 

 • zero of weight was not carried out, the original value of the mass of the 

 sphere was placed on the curve and the times of the other observations 

 on the same sphere were taken from this point. It is very probable 

 that this method of choosing the highest weight has somewhat decreased 

 the accuracy of the calculated constant, for it has been invariably ob- 

 served that a measurable time elapses before a sphere falls into its 

 regular rate of evaporation. It begins slowly, sometimes at not more 

 than half its full rate, and several minutes elapse before it reaches its 

 maximum value. It is probable that better agreement would have 

 been obtained if a point farther along in the observations had been 

 chosen and calculations made in both directions from this. 



It seems clear that for spheres of iodine of mass ranging from 2 milli- 

 grams to very small values, the rate of evaporation is quite accurately 

 proportional to the radius. 



Before taking up any theory of this surprising result it will be best 

 to have data on evaporation from masses having other geometrical 

 shapes, and especially for a flat surface. It is expected that data on 

 these points will be presented to the Academy in the near future. 



Jefferson Physical Laboratory, 

 Cambridge, Mass., 



December, 1909. 



