OF THE LATE MR. WILLIAM STURGEON. 63 



and the disengagement of gas, which is trifling, occurs only 

 when the circuit is complete, and at the surface of the copper 

 plate. The electric powers are highly exalted, and continue 

 in play much longer than with pure zinc ; and the only care 

 of the experimenter is to prevent the copper, or whatever 

 metal be substituted, from becoming amalgamated." 



From the earlier portion of the above extract, it has been 

 alleged that Mr. Sturgeon did not fully perceive the practical 

 superiority of amalgamated, over ordinary zinc plates; but 

 generous and even candid criticism will attribute his reserve 

 to his praiseworthy aim to place before the reader the pro- 

 bable disadvantages as well as the real ascertained advantages 

 of his invention. How much the latter predominated over 

 the former, in Mr. Sturgeon's mind, will be obvious to any 

 one who reads the whole passage. I need hardly remark, 

 that amalgamated zinc plates are at the present day em- 

 ployed in Grove's, Daniell's, and, in short, every form of 

 improved battery. 



1 next notice a memoir on the thermo-magnetism of homo- 

 geneous metals, published in the Philosophical Magazine 

 for 1831, which contains a very laborious investigation of 

 the effect of local heat on simple metals, by which he con- 

 firms Yelin's observation, that currents are produced in any 

 metal by an unequal application of heat, and shows that these 

 currents have a particular reference to the crystalline structure 

 of the metal ; bismuth, antimony, and zinc, which in a pure 

 state exhibit the phenomena in an eminent degree, losing it 

 nearly altogether when their crystalline condition has been 

 destroyed by alliage with a small portion of tin or lead. 

 Professor Thomson has recently discovered that strain, 

 whether arising from crystalline structure or any other cause, 

 produces, according to certain laws, an alteration both in the 

 thermo-electric character of metals, and their power to conduct 

 electrical currents. 



In the year 1825 Arago announced his remarkable dis- 



