320 



Dr. W. W. Pavy on the Physiology of [June 14, 



the filter-plug and dissolved precipitate, are poured into a funnel con- 

 taining a loose plug, to obtain the liquid in a separate form. The 

 requisite washing with distilled water having been performed, there only 

 remains the final stage of the process to be conducted. 



The liquid to be now dealt with contains the copper in the form of 

 nitrate, which experiment has shown to be the most suitable for yielding 

 a pure metallic deposit by galvanic action. For the purpose of col- 

 lecting the deposit, a cylinder of platinum foil soldered to a platinum 

 wire, for hooking on to the negative pole of the battery, is employed. 

 This is immersed in the liquid so as nearly to touch the bottom of the 

 vessel, and inserted within it is a spiral coil of platinum wire, made to 

 form the positive pole of the battery. In order to secure a good con- 

 tinuous connexion, the platinum spiral is closely bound to the copper 

 conducting-wire of the battery, and the other pole is provided with a 

 platinum hook, for the suspension of the cylinder. This precaution has 

 been found necessary from the ready manner in which copper exposed 

 ends become oxidized and rendered imperfect conductors by the oxygen 

 escapiug from the liquid underneath. It may be also mentioned that the 

 platinum spiral, after several days' use, presents a brown surface, and 

 requires to be occasionally cleaned by immersion in hydrochloric acid. 

 In the case of an analysis of blood containing an ordinary amount of 

 sugar, and therefore yielding a limited amount of copper to be deposited, 

 twenty-four hours have been usually found to thoroughly suffice for com- 

 plete removal to occur : but it is necessary that there should be no uncer- 

 tainty upon this point ; and to provide against this the liquid must be 

 tested before the operation is regarded as finished. A small quantity is 

 taken from the bottom of the vessel by means of a pipette, and a little 

 ammonia added to it in a test-tube. Should a decided blue colour be 

 produced it should be returned to the vessel : should no blue colour be 

 perceptible the testing must be carried further. Acetic acid is added in 

 excess to the contents of the test-tube to supersaturate the ammonia, 

 and then a small quantity of a solution of yellow prussiate of potash 

 dropped in. In the absence of copper, nothing but the faint yellow 

 colour of the test is perceptible ; but with the presence of copper a 

 brownish hue is produced, and the galvanic action must be carried on 

 till, on testing, this is no longer brought out. When it is thus found 

 that the whole of the copper has been thrown down, the cylinder is 

 lifted quickly out of the liquid and instantly plunged, first into distilled 

 water and then into spirit. After drying in a water-oven, it is ready for 

 weighing, and it need hardly be said that a delicate balance is required 

 for the purpose. The weight of the cylinder before and after the opera- 

 tion indicates the amount of copper deposited. 



The galvanic action requires to be steadily and continuously main- 

 tained, and a modification of Fuller's mercury-bichromate battery has 

 been found to be highly suitable for use. The arrangement that has been 



