The distinction of crystals into systems according to the degree of symmetry which obtains in them, has been explained elsewhere. Two of these systems, of which the relation as to symmetry might be expressed by saying that one is _square pyramidal_ and the other _oblong pyramidal_, or the first _square prismatic_ and the second _oblong prismatic_, are termed by Mohs, the first, _Pyramidal_, and the second _Prismatic_. And it may {303} be doubted whether it is worth while to invent other terms, though these are thus defective in characteristic significance. As an example of a needless rejection of old terms in virtue of a supposed impropriety in their meaning, I may mention the attempt made in the last edition of Haüy's _Mineralogy_, to substitute _autopside_ and _heteropside_ for _metallic_ and _unmetallic_. It was supposed to be proved that all bodies have a metal for their basis; and hence it was wished to avoid the term _unmetallic_. But the words _metallic_ and _unmetallic_ may mean that minerals _seem_ metallic and unmetallic, just as well as if they contained the element _opside_ to imply this seeming. The old names express all that the new express, and with more simplicity, and therefore should not be disturbed. The maxim on which we are now insisting, that we are not to be too scrupulous about the etymology of scientific terms, may, at first sight, appear to be at variance with our Fourth Aphorism, that words used technically are to retain their common meaning as far as possible. But it must be recollected, that in the Fourth Aphorism we spoke of _common_ words _appropriated_ as technical terms; we here speak of words _constructed_ for scientific purposes. And although it is, perhaps, impossible to draw a broad line between these two classes of terms, still the rule of propriety may be stated thus: In technical terms, deviations from the usual meaning of words are bad in proportion as the words are more familiar in our own language. Thus we may apply the term _Cirrus_ to a cloud composed of filaments, even if these filaments are straight; but to call such a cloud a _Curl cloud_ would be much more harsh. Since the names of things, and of classes of things, when constructed so as to involve a description, are constantly liable to become bad, the natural classes shifting away from the descriptive marks thus prematurely and casually adopted, I venture to lay down the following maxim. {304} APHORISM XIII. _The fundamental terms of a system of Nomenclature may be conveniently borrowed from casual or arbitrary circumstances._