He once wrote a letter on ray-vibrations to Mr. Richard Phillips; at its close he said:--“I think it likely that I have made many mistakes in the preceding pages, for even to myself my ideas on this point appear only as the shadow of a speculation, or as one of those impressions on the mind which are allowable for a time as guides to thought and research. He who labors in experimental inquiries, knows how numerous these are, and how often their apparent fitness and beauty vanish before the progress and development of real natural truth.” “Summing up, then,” says Professor William Stanley Jevons, in “Principles of Science,” “it would seem as if the mind of the great discoverer must combine almost contradictory attributes. He must be fertile in theories and hypotheses, and yet full of facts and precise results of experience. He must entertain the feeblest analogies, and the merest guesses at truth, and yet he must hold them worthless until they are verified in experiment. When there are any grounds of probability he must hold tenaciously to an old opinion, and yet he must be prepared at any moment to relinquish it when a single clear contradictory fact is encountered. ‘The philosopher,’ says Faraday, ‘should be a man willing to listen to every suggestion, but determined to judge for himself. He should not be biassed by appearances; have no favorite hypotheses; be of no school; and in doctrine have no master. He should not be a respecter of persons, but of things. Truth should be his primary object. If to these qualities be added industry, he may indeed hope to walk within the veil of the temple of nature.’” Character, no less than mind of the highest order, ever distinguishes the great researcher. Says Professor Tyndall:--“Those who are unacquainted with the details of scientific investigation, have no idea of the amount of labor expended on the determination of those numbers on which important calculations or inferences depend. They have no idea of the patience shown by a Berzelius in determining atomic weights; by a Regnault in determining co-efficients of expansion; or of a Joule in determining the mechanical equivalent of heat. There is a morality brought to bear upon such matters, which, in point of severity, is probably without a parallel in any other domain of intellectual action.” Surely there was a union of the highest character and of consummate ability in Stas, the Belgian chemist, who eliminated from his chemicals every trace of that pervasive element, sodium, so thoroughly, that even its spectroscopic detection was impossible. A Word for Discursiveness.