Precipitation. 289 



little bill of fifteen pounds. As he distributed his 

 duplicates among his friends, he used to laugh over 

 the story, which he told as a warning never to give 

 an unlimited order." 



On November 4, 1873, Robertson wrote to Dr. R. 

 Brown the following very interesting letter : 



" MY DEAR SIR, 



" A simple way of illustrating the ex- 

 periment of the precipitation of clay in fresh 

 and sea water, is to take two small glass jars of 

 equal size. Test glasses about four inches deep 

 and an inch and three-quarters wide do very well. 

 Fill the two about four-fifths full, the one with 

 sea water, the other with fresh water. Then fill 

 both up with clay dissolved in fresh water, say about 

 the consistence of cream, and stir both well up. Set 

 the jars side by side to settle, and in a very short 

 time the precipitation in the jar containing the sea 

 water will be seen to be going on rapidly, while in 

 the jar with the fresh water little or no change will 

 be observable. That the experiment be successful 

 and true deductions come to, it is necessary to have 

 the fresh water pure, free from salts of any kind, as 

 a very small admixture of these will falsify the 

 experiment to a less or greater extent. This may 

 explain why some lakes, such as the Lake of Geneva, 

 remain clear although muddy glacier water is con- 

 tinually pouring into them. The experiment may 

 be varied by adding different quantities of fresh 

 water to the sea water till it becomes but slightly 

 brackish. Still, with these slightly brackish mixtures 



U 



