156 NAMES AND PROPOSITIONS. 



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 continually modified 

 as scientific knowledge advances, the definitions in the sciences 

 are also constantly varying. A striking instance is afforded 

 by the words Acid and Alkali, especially the former. As 

 experimental discovery advanced, the substances classed with 

 acids have been constantly multiplying, and by a natural con- 

 sequence the attributes connoted by the word have receded and 

 become fewer. At first it connoted the attributes, of combin- 

 ing 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 element in the con- 

 notation of the word. But carbonic acid, silica, sulphurous 

 acid, have no hydrogen in their composition; that property 

 cannot therefore be connoted by the term, unless those sub- 

 stances are no longer to be considered acids. Causticity and 

 fluidity have long since been excluded from the characteristics 

 of the class, by the inclusion of silica and many other sub- 

 stances in it ; and the formation of neutral bodies by com- 

 bination with alkalis, together with such electro-chemical 

 peculiarities as this is supposed to imply, are now the only 

 differentia which form the fixed connotation of the word Acid, 

 as a term of chemical science. 



What is true of the definition of any term of science, is of 

 course true of the definition of a science itself : and accord- 

 ingly, (as observed in the Introductory Chapter of this work,) 

 the definition of a science must necessarily be progressive and 

 provisional. Any extension of knowledge or alteration in the 

 current opinions respecting the subject matter, may lead to a 



