Joseph Henry's genius was attracted primarily to the great experimental problems of his age. Although he was a professor of natural philosophy, a discipline which a century ago combined the various branches of physical science, he is not known to have indulged in the kind of formal considerations to which the title of this evening's discourse alludes. Yet I am confident that my subject is not wholly inappropriate to the occasion of a lecture in honor of Joseph Henry. For his eminent biographer, Charles Greeley Abbot, describes him as “a man of varied culture, of large breadth and liberality of views, of generous impulses and of great gentleness and courtesy of manner”. Hence, while the speculations on which I am about to embark can hardly aspire to honor his memory, we may take comfort in supposing that he would gracefully listen to them and accept them as a small token of respect.