For some time Signor Battelli has been engaged, with remarkable success, in measuring directly the amount of the ‘Thomson effect’ in various metals.

With the exception of iron, the common metals have given him results coinciding as closely as could be expected with those I found in 1872 by an indirect method. Among other particularly satisfactory things, he has directly verified the first of the two changes of sign of the Thomson effect in nickel. And I think it will be allowed that what I introduced long ago as a mere working hypothesis,–that the Thomson effect is directly proportional to the absolute temperature,–if it was not completely established as a fact by my own experiments, has been made absolutely certain by the recent work of Campbell and of Battelli.