82 DISSERTATION SECOND. [paiix r. 



Taking either of the hypotheses, its consequences must 

 be attempted to be traced, supposing a different experi- 

 ment to be made. This must be done with respect to 

 the other hypothesis, and a case will probably at last oc- 

 cur, where the two hypotheses would give different results. 

 The experiment made in those circumstances will furnish 

 an instantia cruris. 



Thus, if the experiment of calcination be performed in 

 a close vessel, and if phlogiston be the cause of the in- 

 crease of weight, it must either escape through the ves- 

 sel, or it must remain in the vessel after separation from 

 the calx. If the former be the case, the apparatus will be 

 increased in weight ; if the latter, the phlogiston must make 

 its escape on opening the vessel. If neither of these be 

 the case, it is plain that the theory of phlogiston is insuf- 

 ficient to explain the facts. 



The experimentum cruris is of such weight in matters of 

 induction, that in all those branches of science where it 

 cannot easily be resorted to (the circumstances of an ex- 

 periment being out of our power, and incapable of being 

 varied at pleasure,) there is often a great want of conclu- 

 sive evidence. This holds of agriculture, medicine, poli- 

 tical economy, &c. To make one experiment, similar to 

 another in all respects but one, is what the experimentum 

 cruris, and, in general, the process of induction, principally 

 requires ; but it is what, in the sciences just named, can 

 seldom be accomplished. Hence the great difficulty of 

 separating the causes, and allotting to each its due propor- 

 tion of the effect. Men deceive themselves in consequence 

 of this continually, and think they are reasoning from fact 

 and experience, when, in reality, they are only reasoning 

 from a mixture of truth and falsehood. The only end an- 

 swered by facts so incorrectly apprehended, is that of mak- 

 ing errour more incorrigible. 



