Those who thus apply principles already discovered are not to be ranked in their intellectual achievements with those who discover new principles; but still, when such applications are masked by the complex relations of space and number, it is impossible not to regard with admiration the clearness and activity of intellect which thus discerns in a remote region the rays of a central truth already unveiled by some great discoverer. 8. As examples in other fields of the application of a scientific discovery to the explanation of natural phenomena, we may take the identification of Lightning with electricity by Franklin, and the explanation of Dew by Wells. For Wells's _Inquiry into the Cause of Dew_, though it has sometimes been praised as an original discovery, was, in fact, only resolving the phenomenon into principles already discovered. The atmologists of the last century were aware[51\3] that the vapour which exists in air in an invisible state may be condensed into water by cold; and they had noticed that there is always a certain temperature, lower than that of the atmosphere, to which if we depress bodies, water forms upon them in fine drops. This temperature is the limit of that which is {239} necessary to constitute vapour, and is hence called the _constituent temperature_. But these principles were not generally familiar in England till Dr. Wells introduced them into his _Essay on Dew_, published in 1814; having indeed been in a great measure led to them by his own experiments and reasonings. His explanation of Dew,--that it arises from the coldness of the bodies on which it settles,--was established with great ingenuity; and is a very elegant confirmation of the Theory of Constituent Temperature. [Note 51\3:_Hist. Ind. Sc._ b. x. c. iii. sect. 5.] 9. As other examples of such explanations of new phenomena by a theory, we may point out Ampère's Theory that Magnetism is transverse voltaic currents, applied to explain the rotation of a voltaic wire round a magnet, and of a magnet round a voltaic wire. And again, in the same subject, when it had been proved that electricity might be converted into magnetism, it seemed certain that magnetism might be converted into electricity; and accordingly Faraday found under what conditions this may be done; though indeed here, the theory rather suggested the experiment than explained it when it had been independently observed. The production of an electric spark by a magnet was a very striking exemplification of the theory of the identity of these different polar agencies.