In the case of discoveries in science or inventions of apparatus, the name of the inventor is very properly employed as the designation. Thus we have the _Torricellian_ Vacuum, the _Voltaic_ Pile, _Fahrenheit's_ Thermometer. And in the same manner with regard to laws of nature, we have _Kepler's_ Laws, _Boyle_ or _Mariotte's_ law of the elasticity of air, _Huyghens's_ law of double refraction, _Newton's_ scale of colours. _Descartes'_ law of refraction is an unjust appellation; for the discovery of the law of sines was made by Snell. In deductive mathematics, where the invention of a theorem is generally a more definite step than an induction, this mode of designation is more common, as _Demoivre's_ Theorem, _Maclaurin's_ Theorem, _Lagrange's_ Theorem, _Eulerian_ Integrals. In the _History of Science_[41\4] I have remarked that in the discovery of what is termed galvanism, Volta's {307} office was of a higher and more philosophical kind than that of Galvani; and I have, on this account, urged the propriety of employing the term _voltaic_, rather than _galvanic_ electricity. I may add that the electricity of the common machine is often placed in contrast with this, and appears to require an express name. Mr. Faraday calls it _common_ or _machine_ electricity; but I think that _franklinic_ electricity would form a more natural correspondence with _voltaic_, and would be well justified by Franklin's place in the history of that part of the subject. [Note 41\4: b. xiii. c. 1.] APHORISM XIV. _The Binary Method of Nomenclature_ (_Names by Genus and Species_) _is the most convenient hitherto employed in Classification._ THE number of species in every province of Natural History is so vast that we cannot distinguish them and record the distinctions without some artifice. The known species of plants, for instance, were 10,000 in the time of Linnæus, and are now probably 60,000. It would be useless to endeavour to frame and employ separate names for each of these species. The division of the objects into a subordinated system of classification enables us to introduce a Nomenclature which does not require this enormous number of names. The artifice employed is, to name a specimen by means of two (or it might be more) steps of the successive division. Thus in Botany, each of the Genera has its name, and the species are marked by the addition of some epithet to the name of the genus. In this manner about 1,700 Generic Names, with a moderate number of Specific Names, were found by Linnæus sufficient to designate with precision all the species of vegetables known at his time. And this _Binary Method_ of Nomenclature has been found so convenient, that it has been universally adopted in every other department of the Natural History of organized beings. {308}