498 Prof. Richards and Mr. Archibald : A Study of 



Fig. 16. — Diagram representing Rate of Increase in Crystal 

 Diameter. 



Time from birth of crystal 



Q represents point from average of crystals taken at random. 

 •0 represents point from average of selected crystals. 



represents probable actual rate of growth. 



represents equation D 3 =kt. 



represents equation D 2 =kt. 



The unit of time is the time of one revolution of the shutter, or 1*25 seconds. 

 The substance was potassic iodide. 



■either almost as large as the second image, or very small 

 compared with it. It will appear almost as large as the 

 second image when the preceding exposure has just not 

 cauo-ht the beginning of the crystal, which has thus had 

 a whole interval for growth ; or very much smaller than the 

 second image when the first impression has registered a 

 crystal only a very small fraction of a second old. Marked 

 examples of the former case are to be found in fig. 11, and 

 of the latter in the largest crystal in fig. 9, and the smallest 



