Distribution of Copper Compounds 153 



lowing table of solubilities of various compounds of copper in 

 different solvents, made up from different sources of information. 

 The exact determinations of solubility by E. E. Free and W. H. 

 Ross were made to obtain data needed in this investigation. 



This table indicates that the carbonates and the silicate 

 (chrysocolla) of copper, which are the compounds in which the 

 metal must largely occur in the soil, are notably soluble in 

 aqueous solutions of carbon dioxide.^ Large amounts of sodium 

 chloride and sodium sulphate increase the solubility of precipi- 

 tated basic copper carbonate. In pure water, copper compounds, 

 so far as observed, are but slightly soluble. Fluctuations in the 

 content of carbon dioxide and of soluble salts in soil waters, and 

 variations in the character of the soluble salts, are shown to 

 affect the copper content of such waters. 



In brief, the final effect upon plant roots of copper in the soil 

 is the complex resultant of many opposing influences tending, on 

 the one hand, to remove copper from solution, and, on the other, 

 to maintain it in toxic soluble form. Observations on Hip soil 

 usually fail to give satisfactory evidence as to the toxic or non- 

 toxic effects to be expected from small percentages of copper that 

 may be present. Direct chemical and physiological studies of 

 plants afford much more satisfactory information. This mode of 

 attack has been employed considerably in this investigation. 



In view of the general tendency in nature to hinder the move- 

 ments of copper in soils and to convert it into its insoluble forms, 

 and independently of any tendency of the plant itself to assimi- 

 late or to reject copper, we should expect to find relatively small 

 amounts of this element in plant tissues. 



The following analytical determinations of copper in ores 

 and tailings were made in samples carefully collected by the 

 writer throughout the district studied. In all cases, the copper 

 was determined electrolytically, manipulations of great delicacy 

 having been developed for the determination of the minute 

 amounts of copper often encountered. A full statement of the 

 methods of preparing samples for analysis, and of determining 



3 Sullivan has shown that powdered silicates react with copper sulphate 

 to withdraw copper from solution; and that this copper will then be redis- 

 solved by a solution of potassium sulphate. U. S. Geol. Survey, Bull. 

 312, 1907. 



