Conducting Powers of Metals at Different Temperatures. 185 



hydric acid and adding excess of ammonia; the platinum then re- 

 mains in solution while the iron is precipitated as oxyd. Alu- 

 mina has not yet been detected with certainty in the ashes of 

 organic substances. The assertion of Presenilis that phosphate 

 of peroxyd of iron cannot be perfectly decomposed by fusion 

 with an alkaline carbonate is, according to Rose, incorrect. 





Art. XIV. — Abstract of an Article on the Conducting Powers of 

 the Metals at different temperatures, fyc. ; by M. Edmond 



BeCQUEREL.* 



After an introductory view of what has been done by oth- 

 ers on the subject of conducting powers, the author proceeds to 

 introduce a description of the apparatus employed by himself in 

 his researches on the conducting powers of the metals, remarking 

 that it is similar to that employed by the elder Becquerel, but 



has some modifications which he thinks frees it from all source 

 of err 



:or. 



The galvanometer employed was a differential instrument, 

 having two separate wires in its coil, these wires being first cov- 

 ered with silk and twisted together into a cord, after the manner 

 of Poggendorff, before being coiled on the frame of the galvan- 

 ometer. If these two wires be made parts of two distinct metal- 

 lic conductors of equal conducting power connecting the poles of 

 the same galvanic pair or battery, and so that the two currents 

 shall be in opposite directions through the galvanometer, the 

 needle of the Latter will be stationary, and this is the test of the 

 equality of the conducting powers of the two conductors. Of one 

 of these conductors, Wheatstone's rheo-stat is made to form a part 

 of the other wire to be submitted to experiment, so that if any ad- 

 dition be made to the length of the latter included in its circuit, a 

 corresponding addition must be made to the length of the wire 

 of the rheo-stat included in its circuit, in order to maintain the 

 equilibrium of the needle. Thus if after the needle has been 

 brought to rest, one additional metre of the wire of experiment be 

 fought into the circuit, it will be necessary to turn the rheo-stat 

 till such an additional length of its wire is brought into the cir- 

 cuit to which it belongs, as is equivalent in its resistance to con- 

 duction to the metre of the wire of experiment Thus the rheo- 

 stat is made to furnish a measure of the resistance to conduction 

 °f a given length of wire. The apparatus was arranged so that 

 scarce any change was made hi either circuit, beyond that of the 





* From the Ann. <lo Oh. el de Ph., 3.1 series, vol. xvii, p. 242. 

 Second Sheiks, Vol. VIII, No. 23.— Sept., 1849. 24 



