In later times, the limitation imposed upon a word by its appropriation to scientific purposes, is often more marked than in the cases above described. Thus the _variation_ is made to mean, in astronomy, the second inequality of the moon's motion; in magnetism, the _variation_ signifies the angular deviation of the {261} compass-needle from the north; in pure mathematics, the _variation_ of a quantity is the formula which expresses the result of any small change of the most general kind. In like manner, _parallax_ (παράλλαξις) denotes a _change_ in general, but is used by astronomers to signify the change produced by the spectator's being removed from the center of the earth, his theoretical place, to the surface. _Alkali_ at first denoted the ashes of a particular plant, but afterwards, all bodies having a certain class of chemical properties; and, in like manner, _acid_, the class opposed to alkali, was modified in signification by chemists, so as to refer no longer to the taste. Words thus borrowed from common language, and converted by scientific writers into technical terms, have some advantages and some disadvantages. They possess this great convenience, that they are understood after a very short explanation, and retained in the memory without effort. On the other hand, they lead to some inconvenience; for since they have a meaning in common language, a careless reader is prone to disregard the technical limitation of this meaning, and to attempt to collect their import in scientific books, in the same vague and conjectural manner in which he collects the purpose of words in common cases. Hence the language of science, when thus resembling common language, is liable to be employed with an absence of that scientific precision which alone gives it value. Popular writers and talkers, when they speak of _force_, _momentum_, _action and reaction_, and the like, often afford examples of the inaccuracy thus arising from the scientific appropriation of common terms.