126 Dr. Percy and Mr. R. Smith on the Detection of Gold 



must occur when the plant comes in contact with the water of 

 the earth. But it may be proved, that in an aqueous solution 

 composed of successive layers gradually increasing in concenti'a- 

 tion, the sum of all the excitations which occur at the places of 

 contact of the different layers is less than the excitation between 

 pure water and the most concentrated of the layers ; and thus, 

 no matter what the nature of the solution may be, pure water 

 will be positively electric at the place of transition to the most 

 concentrated layer of the solution. 



Now this is the exact deportment of plants in the experiments 

 described above. The epidermis forms a sharp and permanent 

 limit between the sap in the highest possible state of concen- 

 tration behind it, and the pure water on its exterior, while by 

 means of the roots a very gradual transition to water is effected. 

 In like manner the sap exuding from a wound into the sur- 

 rounding water can spread itself with gi-adually diminishing con- 

 centration. 



The electroscopic actions of the electricity of plants correspond 

 to the origin of the latter, as explained above; they are very 

 feeble. Even by means of the condenser and dry-pile electro- 

 scope, no electric charges of sufficient magnitude can be obtained 

 to enable us to attribute them in a certain manner to the plant 

 as som-ce. By forming a compound circuit of plants, I suc- 

 ceeded in obtaining stronger tension effects. Each uninjured 

 leaf, with its severed end, forms as it were a galvanic element. It 

 was therefore only necessary to arrange suitably a number of 

 such leaves in a series to obtain a compound galvanic circuit. 

 Thus, with a battery of twelve sappy leaves, a tension was obtained 

 which amounted to somewhat more than half of that exhibited 

 by an element of zinc and copper immersed in water. 



XVIII. On the Detection of Gold in Lead and its Compounds. By 

 John Percy, M.D., F.R.S., Lecturer on Metallurgy at the 

 School of Mines, Jermyn Street ; and Richard Smith, Assist- 

 ant in the Metallurgical Laboratory*. 



IN the Philosophical Magazine for April 1853, it was stated 

 that gold had been detected in numerous samples of lead 

 and its compounds met with in commerce, and that the experi- 

 ments upon which the statement was founded should appear on 

 a future day. Those experiments are now given. The investi- 

 gation is still in progress, and will embrace the examination of a 

 variety of ores. 



All the experiments have been made by Mr. Smith, and the 



* Communicated by the Authors. 



