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principle on which many of the practical applications of 

 electricity were based. He then explained the process of 

 assaying copper and other ores by electricity, a process he 

 himself invented some years ago, and which is now in very 

 general use, and is so simple that the most unlearned can 

 avail themselves of it. The following is the process : — 

 Suppose we wish to assay a copper ore, dissolve 400 grains 

 in acid, in the usual manner, dilute with water, and put into 

 a glass or earthen jar ; then take a strip of copper plate, 

 about one inch wide and two inches long, having a wire six 

 inches long soldered to it ; procure also an iron plate of a 

 similar size, having a wire soldered to it ; cover the iron 

 plate with brown paper, and weigh the copper plate with 

 great exactness ; then twist the wires together, and plunge 

 the two plates into the dilute solution of the ore; allow 

 them to remain there until all action ceases ; then free 

 the copper plate from the iron, dry it, and re-weigh it, and 

 the excess over the former weight will be the exact amount 

 of copper contained in the 400 grains of ore, because the 

 axiom mentioned has come into play ; the solution of metal 

 is decomposed by the electric current generated by the two 

 plates or galvanic pair ; the oxygen and acid have gone to 

 the iron plate (the positive pole), while the metal copper 

 has gone to the copper plate (the negative pole), and on 

 this plate it will be seen in a bright metallic state. 



A variation of this process, but based on the same law, 

 is the electrotype and electro-plating process; the object 

 to be covered forms the negative pole of a galvanic battery 

 or pair ; and this object being plunged into a metallic solu- 

 tion, — copper, silver, or gold, — is in a short time covered 

 by the metal. The whole of these processes were shown by 

 Mr. Roberts. He then explained the principle of preserving 

 metals by electric action; for example — Sir H, Davy's 

 attempt to preserve the sheathing of ships. He next 

 touched upon the mechanical preservation, such as what 



