TRAINS OP REASONING. 247 



continually take place in the less advanced branches of physical 

 knowledge, without enabling them to throw off the character of 

 experimental sciences. Thus with regard to the two uncon- 

 nected propositions before cited, namely, Acids redden vege- 

 table blues, Alkalies make them green ; it is remarked by 

 Liebig, that all blue colouring matters which are reddened by 

 acids (as well as, reciprocally, all red colouring matters which 

 are rendered blue by alkalies) contain nitrogen : and it is quite 

 possible that this circumstance may one day furnish a bond of 

 connexion between the two propositions in question, by show- 

 ing that the antagonistic action of acids and alkalies in pro- 

 ducing or destroying the colour blue, is the result of some 

 one, more general, law. Although this connecting of detached 

 generalizations is so much gain, it tends but little to give a 

 deductive character to any science as a whole ; because the new 

 courses of observation and experiment, which thus enable us 

 to connect together a few general truths, usually make known 

 to us a still greater number of unconnected new ones. Hence 

 chemistry, though similar extensions and simplifications of its 

 generalizations are continually taking place, is still in the main 

 an experimental science ; and is likely so to continue unless 

 some comprehensive induction should be hereafter arrived at, 

 which, like Newton's, shall connect a vast number of the 

 smaller known inductions together, and change the whole 

 method of the science at once. Chemistry has already one 

 great generalization, which, though relating to one of the sub- 

 ordinate aspects of chemical phenomena, possesses within its 

 limited sphere this comprehensive character ; the principle of 

 Dalton, called the atomic theory, or the doctrine of chemical 

 equivalents : which by enabling us to a certain extent to fore- 

 see the proportions in which two substances will combine, 

 before -the experiment has been tried, constitutes undoubtedly 

 a source of new chemical truths obtainable by deduction, as 

 well as a connecting principle for all truths of the same de- 

 scription previously obtained by experiment. 



7. The discoveries which change the method of a 

 science from experimental to deductive, mostly consist in 



