FALLACIES OF OBSERVATION. 395 



the whole superstructure of a theory has been founded. 

 As, in the cases hitherto examined, a general propo- 

 sition was too rashly adopted, on the evidence of par- 

 ticulars, true indeed, but insufficient to support it; 

 so in the cases to which we now turn, the particulars 

 themselves have been imperfectly observed, and the 

 singular propositions upon which the generalization 

 is grounded, or some at least of those singular propo- 

 sitions, are false. 



Such, for instance, was one of the mistakes com- 

 mitted in the celebrated phlogistic theory; a doctrine 

 which accounted for combustion by the extrication of 

 a substance supposed to be contained in all com- 

 bustible matter, and to which the name phlogiston 

 was given. The hypothesis accorded tolerably well 

 with superficial appearances: the ascent of flame na- 

 turally suggests the escape of a substance; and the 

 visible residuum of ashes, in bulk and weight, generally 

 falls extremely short of the combustible material. 

 The error was, non-observation of an important por- 

 tion of the actual residue, namely, the gaseous pro- 

 ducts of combustion. When these were at last 

 noticed and brought into account, it appeared to be 

 an universal law, that all substances gain instead of 

 losing weight by undergoing combustion; and, after 

 the usual attempt to accommodate the old theory to 

 the new fact by means of an arbitrary hypothesis 

 (that phlogiston had the quality of positive levity 

 instead of gravity), chemists were conducted to the 

 true explanation, namely that instead of a substance 

 separated, there was on the contrary a substance 

 absorbed. 



Many of the absurd practices which have been 

 deemed to possess medicinal efficacy, have been in- 

 debted for their reputation to non-observance of some 



