306 



effect of the conductivity of one piece of solid copper, I have not 

 yet found whether or not specimens either good or bad retain their 

 specific qualities after melting. 



I may add, that it will be of great importance to ascertain the laws 

 of variation of conductivity with temperature, of different specimens 

 of nearly pure copper differing largely in conductivity. I have 

 hitherto used standards of copper wire in all the relative determina- 

 tions of conductivity which I have made for different commercial 

 specimens and artificial alloys of copper ; and before I found the 

 very large differences of conductivity shown in this and in my pre- 

 ceding communication to the Royal Society (June 15, 1857), it 

 seemed natural to suppose that the relation between specimen and 

 standard would remain constant, or nearly constant, when the tem- 

 peratures of the two are varied to the same extent. Now, however, 

 it seems scarcely probable that this can be the case, and a rigorous 

 experimental examination of the influence of temperature becomes 

 necessary. 



P.S. April 11, 1860. I append the following extract from 

 evidence which I gave on examination before the Government Com- 

 mittee on submarine telegraphs on the 17th December, 1859, as it 

 bears directly on the subject of the preceding article, and shows 

 what degree of weight may in my opinion be attached to the syn- 

 thetical attempts which have been described. 



(Chairman.) Question 2458. Soon after you became a Director 

 of the Atlantic Telegraph Company, was your attention directed to 

 the conductivity of copper ? Yes. 



2459. You instituted a series of experiments, did not you, to deter- 

 mine the variation of this quality in different samples of copper ? 

 A number of samples of copper were, at my request, put into my 

 hands for the purpose of measuring their conductivity in consequence 

 of my having accidentally noticed differences greater than I expected 

 in the conducting power of one or two samples which I had had 

 previously. 



2460. Will you be good enough to state the general results at 

 which you ultimately arrived, arid your modes of experimenting ? 

 My modes of experimenting did not differ materially from the 

 methods which had been followed by certain other experimenters, 



