This scientific fixation of the meaning of words is to be looked upon as a matter of convention, although it is in reality often an inevitable result of the progress of science. _Momentum_ is conventionally defined to be the product of the numbers expressing the weight and the velocity; but then, it could be of no use in expressing the laws of motion if it were defined otherwise. Hence it is no valid objection to a scientific term that the word in common language does not mean exactly the same as in its common use. It is no sufficient reason against the use of the term _acid_ for a class of bodies, that all the substances belonging to this class are not sour. We have seen that a _trapezium_ is used in geometry for any four-sided figure, though originally it meant a figure with two opposite sides parallel and the two others equal. A certain stratum which lies below the chalk is termed by English geologists _the green sand_. It has sometimes been objected to this denomination that the stratum has very frequently no tinge of green, and that it is often composed of lime with little or no sand. Yet the term is a good technical term in spite of these apparent improprieties; so long as it is carefully applied to that stratum which is geologically equivalent to the greenish sandy bed to which the appellation was originally applied. When it appeared that _geometry_ would have to be employed as much at least about the heavens as the earth, Plato exclaimed against the folly of calling the {281} science by such a name; since the word signifies 'earth-measuring;' yet the word _geometry_ has retained its place and answered its purpose perfectly well up to the present day. But though the meaning of the term may be modified or extended, it must be rigorously fixed when it is appropriated to science. This process is most abundantly exemplified by the terminology of Natural History, and especially of Botany, in which each term has a most precise meaning assigned to it. Thus Linnæus established exact distinctions between _fasciculus_, _capitulum_, _racemus_, _thyrsus_, _paniculus_, _spica_, _amentum_, _corymbus_, _umbella_, _cyma_, _verticillus_; or, in the language of English Botanists, _a tuft_, _a head_, _a cluster_, _a bunch_, _a panicle_, _a spike_, _a catkin_, _a corymb_, _an umbel_, _a cyme_, _a whorl_. And it has since been laid down as a rule[20\4], that each organ ought to have a separate and appropriate name; so that the term _leaf_, for instance, shall never be applied to _a leaflet_, _a bractea_, or _a sepal_ of the calyx. [Note 20\4: De Candolle, _Theor. El._ 328.] Botanists have not been content with fixing the meaning of their terms by verbal definition, but have also illustrated them by figures, which address the eye. Of these, as excellent modern examples, may be mentioned those which occur in the works of Mirbel[21\4], and Lindley[22\4]. [Note 21\4: _Élémens de Botanique_.] [Note 22\4: _Elements of Botany_.] APHORISM VI.