192 NAMES AND PROPOSITIONS. 



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 conse- 

 quence 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 element in the connotation 

 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 substances 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 substances in it : 

 and the formation of neutral bodies by combination 

 with alkalis, together with such electro- chemical pecu- 

 liarities 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. 



Scientific men are still seeking, and may be long 

 ere they find, a suitable definition of one of the 

 earliest words in the vocabulary of the human race, 



