In like manner, if we think of the discoveries of Sir Humphrey Davy—their nature and relations to physiology as well as chemistry,—we see how much there might have been that was preparatory, and, to a mind like Davy's, suggestive, in the investigations of preceding and contemporaneous philosophers. Priestly had discovered oxygen gas; Galvani and Volta had shown those remarkable phenomena which constitute that important branch of knowledge, "Voltaic electricity;" Berzelius had effected the decomposition of certain salts by the Voltaic pile; and Lavoisier had even predicted as _probable_ what Davy was destined to demonstrate[3] In medical science, few things have been more talked of than the discovery of the circulation of the blood. Now it is curious to observe that every fact essential to the demonstration of it had been made out by previous investigators[4] but no one had deduced from them the discovery of the circulation until Harvey, although it was a conclusion scarcely more important than obvious. There is surely something very encouraging in the reflection, that the advance of knowledge thus results from the accumulated labours of successive minds. It suggests, that however unequally the honours may appear to be distributed—however humble, in our eyes, the function of those who unconsciously prepare the way to great discoveries,—still it may involve a duty no less important than the more lofty mission of enunciating them. The importance of a man's mission can never be estimated by human judgment. We can never know the mission; still less its relations to the power, or the temptations by which that power has been assailed. The most humble may here often approach as nearly to his duty, as the most gifted may have fallen short of it. Our faculties cannot penetrate the matter. We often see men placed in positions for which they appear wholly unfitted—men who seem to be bars to that progress which we should fancy it their duty to promote. Again, we observe that almost all great discoveries have to encounter opposition, persecution, obloquy, or derision; and when they are established, a host of claimants rise up to dispute the property with the rightful owner. A man who is in earnest cares little for these things. They may at times discourage and disappoint him; but they only strengthen his faith, that a day will come when an unerring justice will accord to every useful improvement its proper place and distinction.