CHAPTER VII 



ADDITIONAL EXPERIMENTS MADE WITH PURE COL- 

 LOIDAL SILICA PREPARED BY GRAHAM'S METHOD 



AFTER mentioning, towards the end of last June, 

 to Dr, Otto Rosenheim, the lecturer on Physi- 

 ological Chemistry at King's College, the diffi- 

 culties I had had to contend with owing to the 

 varying composition of the sodium silicate of com- 

 merce, he kindly gave me a small quantity of pure, 

 but dilute, colloidal silica. Graham says 1 this is 

 1 easily obtained in a state of purity, but it can- 

 not be preserved. It may remain fluid for days 

 or weeks in a sealed tube, but it is sure to gelatin- 

 ise and become insoluble at last." When I wrote 

 to Dr. Rosenheim in reference to this point, and 

 to some of the results which I had obtained with 

 his solution, his reply was as follows : The solu- 

 tion is, as you assume, very dilute. It has been 

 prepared according to Graham's directions, dia- 

 lysed but not concentrated, and seems to keep 



l Trans., 1861, p. 183. 



78 



