DKFIMTION. 97 



not 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 fonnation of neutral 

 bodies by combination with alkalis, together with such electro-chemi- 

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

 entia; 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 word^ in the vocabulary of the 

 human race, and one of those of which the popular sense is plainest 

 and best understood. The word I mean is Heat ; and the source of 

 the difficulty is the imperfect state of our scientific knowledge, which 

 has shown to us multitudes of phenomena certainly coimected with the 

 same power which is the cause of what our senses recognize as heat, 

 but has not yet taught us the laws of those phenomena with sufficient 

 accuracy to admit of our determinhig under what characteristics the 

 whole of those phenomena shall ultimately be embodied as a class : 

 which characteristics would of course be so many diffisrentice for the 

 definition of the power itself AVe have advanced far enough to know- 

 that one of thee attributes connoted must be that of operating as a 

 repulsive force : but this is certainly not all which must ultimately be 

 included in the scientific d.efinition of heat. 



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

 true of the definition of a science itself: and accordingly, we showed 

 in the Introductory Chapter of this work, that the definition of a science 

 must necessarily be progi'essive and provisional. Any extension of ' 

 knowledge, or alteration in the current opinions respecting the subject 

 matter, may lead to a change more or less extensive in the paiticulars 

 included in the science ; and its composition being thus altered, it may / 

 easily happen that a different set of characteristics will be found better 

 adapted as differentiae for defining its name. 



In the same manner in which, as we have now shown, a special or 

 technical definition has for its object to expound the artificial classi- 

 fication out of which it grows ; the Aristotelian logicians seem to have 

 imagined that it was also the business of ordinary definition to expound 

 the ordinary, and what they deemed the natural, classification of things, 

 namely, the division of them into Kinds ; and to show the place which 

 each Kind occupies, as superior, collateral, or subordinate, among 

 other Kinds. This notion would account for the rule that all defi- 

 nition must necessarily be per genus et differentiam, and would also 

 explain why any one differentia was deemed sufficient. But to 

 expound, or exjiress in words, a distinction of Kind, has already been 

 shown to be an impossibility : the very meaning of a Kind is, that tin; 

 properties which distinguish it do not grow out of one another, and 

 cannot therefore be set forth in words, even by implication, othei"wise 

 than by enumerating them all : and all are not known, nor ever will be 

 60. It is idle, therefore, to look to this as one of the pui-poses of a 

 definition : while, if it be only required that the definition of a Kind 

 should indicate what Kinds hiclude it or are included by it, any defi- 

 nitions which expound the connotation of the names will do this : for 

 the name of each class must necessarily connote enough of its proper- 

 ties to fix the boundaries of the class. If the definition, therefore, is 

 N 



