LABORATORY NOTES. 329 



To ascertain if all the copper has been deposited the sides of the 

 beaker are washed down with hot water, and the whole left one 

 hour; if more copper then comes down on the previously clean 

 portion of the cylinder a little more water is added, and so on until 

 no more copper is deposited. The solution is drawn off with a 

 syphon, and the solution gradually replaced Avith water whilst the 

 current is still passing (beaker twice filled is generally sufficient) ; 

 finally sufficient water is left in the beaker to cover the cylinder. 

 The cylinder and spiral are now disconnected from battery, the 

 cylinder removed from beaker and washed with hot water, then 

 with alcohol, and dried by^ burning off the alcohol. The cylinder 

 is hung in balance case for twenty minutes and weighed. The 

 cylinders vary but little in weight; they lose slightly in use, and are 

 in practice only weighed clean about once a week. 



The following duplicate determinations made exactly as above 

 show that the process admits of considerable accuracy: — 



P, I 14-190 .' ( ol-6o ■( I 19-045 \ j 21-12 i ) 17-4(1.5 ( 



ures.... ^ j^.j.- ^ 1,51.711 \ 19-045 I ) 24-11 j U7-345 ! 



Electrolytic copper taken, -1127 ; found, -1126. 



The presence of bismuth to an extent of 1 per cent, calculated 

 on the amount of copper in solution is indicated by a darkening 

 of the deposited copper. With larger quantities of bismuth 

 present the deposit is dark and sjjongy, and cannot be washed 

 without loss. In such cases the method is modified as follows: — 

 The sohition of ore is made with aqua regia, and evaporated to 

 dryness, taken up with hydrochloric acid, filtered and insoluble 

 residue washed with hot water, filtrate precijjitated with 

 sulphuretted hydrogen, precipitated sulphides filtered, washed 

 and oxidised with smallest possible quantity 1 — 1 nitric acid; 

 solution nearly neutralised with ammonia, ammonium carbonate 

 in slight excess added, and then a snail quantity of ammonia 

 warmed, and allowed to stand to cool ; precipitate filtered off, 

 washed, and filtrate made slij^htly acid with nitric acid, and 

 evaporated to convenient bulk for plating out. 



Arsenic and Antimony .—Oi these the former is not precipitated 

 until all the copper is removed, but antimony, according toHampe, 

 is deposited to a small extent together with the copper ; in «ny 

 casi; their removal is necessary if present to extent of more than 

 0-05 per cent, on the ore. We accomplish this by precipitating 

 the copper in a caustic soda solution by means of sulphuretted 

 hydrogen. The precipitate is filtered off from the solution 

 containing arsenic and antimony dried, oxidised with nitric acid, 

 evaporated just dry, taken up with water, and plated out. 



Selenium and tellurium, if present, may be removed in a similar 

 manner. 



Silver, if in large quantity, is precipitated as chloride and 

 filtered off, and the solution evaporated with sulphuric acid as 

 before; if in but small quantity — less than loz.to the ton of ore — 



