Flotation of Minerals, Part II. 



3)5 



the law of gaseous diffusion apparently not being obeyed. With 

 the view of further investigating these gas adsorption pheno- 

 mena, the following series of experiments were conducted: — 



1. Pieces of copper and silver foil were cleaned by boiling 

 in a solution of sodium hydroxide, washing in distilled water 

 and drying. These cleaned pieces of foil and similar uncleaned 

 pieces of foil were then placed in a vacuum flask together with 

 (a) ordinary distilled water, (b) air-free distilled water, (c) 

 ordinary tap water, and subjected to a reduced pressure (26 

 inches mercury), with the following results: — 



Distilled Water. 



Air free Distilled 

 Water. 



Tap Water. 



Cleaned Foil 



A few bubbles collected 

 on the foil, but the 

 foil does not float. 



Very few bubbles 

 formed on the 

 foil ; when these 

 were detached 

 no more formed. 



Foil floats by 

 the buoyan- 

 cy of the 

 bubbles that 

 become at- 

 tached. 



Uncleaned Foib 



Many bubbles become 

 attached and the foil 

 floats to the surface. 



A few bubbles form 

 on the foil, on 

 detaching which 

 no more formed. 



Same as clean- 

 ed foil. 



On repeating these experiments with the various waters to 

 which a drop of oleic acid had been added and agitated, in all 

 cases the cleaned and uncleaned foil collected gas bubbles. The 

 uncleaned foil floated and remained floating, but the cleaned 

 foil came to the surface repeatedly but did not remain floating. 

 These experiments show that the persistency of the attachment 

 of the gas bubbles is increased when the surface of the foil is 

 contaminated with some oily substance. 



2. Six steel needles were boiled in strong sodium hydroxide 

 solution for half an hour, washed with distilled water and al- 

 lowed to stand in absolute alcohol for half an hour. They Avere 

 then dried in a desiccator. The needles would not then float on 

 freshly drawn distilled water. When the water was allowed 

 to stand for some time in an evaporating dish, exposed to the 

 air, it was found that they would float. 



9a 



