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U. S. BUREAU OF FISHERIES 



heated very gentl}^ until the action of the nitric acid ceased, after 

 which they were heated more strongly until digestion was complete 

 and SO3 was evolved. 



The solution at this point became nearly colorless. Samples con- 

 taining much copper of course would color the acid slightly blue ; and 

 those which contained siliceous material or much mud would have a 

 slight sediment remaining when digestion was complete. The solution 

 should not show a yellow or brown color. If it did, it was cooled and 



4 



WEEKS 



Figure 10. — Copper content of treated twine 



treated with concentrated nitric again, and the process repeated until 

 the solution became colorless or blue-green. After a colorless solution 

 was obtained, it was cooled and 5 or 10 cubic centimeters of water 

 added so as to dissociate the nitrosylsulphuric acid which was formed 

 from the decomposition of nitric acid, and about 5 cubic centimeters 

 of bromine water was added to destroy nitrous acid. The flask was 

 then heated until the solution was evaporated to small volume and 

 white fumes of SO3 were given off. The flask was allowed to cool, the 



