96 NAMES AND PROPOSITIONS. 



"What would otherwise be a mere description, may be raised to the 

 rank of a real definition by the peculiar purpose which the speaker or 

 "Writer has in view. As was seen in the preceding chapter, it may, for 

 the ends of a particular art or science, or for the more convenient 

 statement of an author's particular views, be advisable to give to some 

 general name, without altering its denotation, a special connotation, 

 different from its ordinary one. When this is done, a definition of the 

 name by means of the attributes which make up the special connota- 

 tion, though in genqral a mere accidental definition, or description, 

 becomes on the particular occasion and for the particular purpose, a 

 complete and genuine definition. This actually occurs with respect 

 to one of the preceding examples, " Man is a maramiferous animal 

 having two hands," which is the scientific definition of man con- 

 sidered as one of the species in Cuvier's distribution of the animal 

 kingdom! 



In cases of this sort, although the definition is still a declaration of 

 the meaning which in the particular instance the name is appointed to 

 convey, it cannot be said that to state the meaning of the word is the 

 purpose of the definition. The purpose is not to expound a name, but 

 to help to expound a classification. The special meaning which Cuvier 

 assigned to the word Man (quite foreign to its ordinary meaning,, 

 though involving no change in the denotation of the word), was inci- 

 dental to a plan of arranging animals into classes on a certain principle, 

 that is, according to a certain set of distinctions. And since the defi- 

 nition of Man according to the ordinary connotation of the word, though 

 it would have answered every other purpose of a definition, would not 

 have pointed out the place Avhich the species ought to occupy in that 

 particular classification ; he gave the word a special connotation, that 

 he might be able to define it by the kind of attributes upon which, for 

 reasons of scientific convenience, he had resolved to found his division 

 of animated nature. 



Scientific definitions, whether they are definitions of scientific terms ' 

 or of common terms, used in a scientific sense, are almost always of the 

 kind last spoken of: .their main purpose is to serve as the landmarks 

 of scientific classification. And since the classifications in any science 

 are continua,lly rnodified as' scientific knowledge iadvances, the defi- 

 nitions in the sciences are also constantly varying. A striking mstance 

 is afforded by the words Acid and Alkali, especially the foniier. As 

 experimental discovery advanced, the substances classed with acids 

 have been constantly multiplying, and by a natural consequence the 

 attributes connoted by the word have receded and become fewer. At 

 first it connoted the attributes, of combining with an alkali to form a 

 neutral substance (called a Salt) ; being compounded of a base and 

 oxygen; causticity to the taste and touch; fluidity, &c. The true 

 analysis of muriatic acid, into chlorine and hydrogen, caused the second 

 property, composition from a base and oxygen, to be excluded from 

 the connotation. The same , discovery fixed the attention of chemists 

 upon hydrogen as an important element in acids ; and more recent 

 discoveries having led to the recognition of its presence in sulphuric, 

 nitric, and many other acids, where its existence was not previously 

 suspected, there is now a tendency to include the presence of this ele- 

 ment in the connotation of the word. But carbonic acid, silica, sulphu- 

 rous acid, have no hydrogen in their composition ; that property can- 



