THE USE OF HYPOTHESIS. 149 



with so many experimental facts as to have great appear- 

 ance of truth. Under such circumstances we have need 

 of some new experiment, which shall give results agreeing 

 with one hypothesis hut not with the other. 



Any such experiment which decides between two rival 

 theories may be called an Experimentum Crucis, an 

 Experiment of the Finger Post. Whenever the mind 

 stands, as it were, at cross-roads, and knows not which 

 way to select, it needs some decisive guide, and Bacon 

 therefore assigned great importance and authority to in- 

 stances or facts which serve in this capacity. The name 

 given by Bacon has become exceedingly familiar ; it is 

 perhaps almost the only one of Bacon's figurative expres- 

 sions which has passed into common use. We even find 

 Newton, as I have already mentioned, using the name 

 (vol. ii. p. 1 34). 



I do not think, indeed, that the common use of the 

 word at all agrees with that intended by Bacon. Sir 

 John Herschel says that ' we make an experiment of the 

 crucial kind when we form combinations, and put in action 

 causes from which some particular one shall be deliberately 

 excluded, and some other purposely admitted 10 / This, 

 however, seems to be the description of any special ex- 

 periment not made at haphazard. Pascal's experiment 

 of causing a barometer to be carried to the top of the 

 Puy-de-D6me has often been considered as a perfect 

 experimentum cn/cis, if not the first distinct one on 

 record"; but if so, we must dignify the doctrine of 

 Nature's abhorrence of a vacuum with the position of a 

 rival theory. A crucial experiment must not simply 

 confirm one theory, but must negative another ; it must 

 decide a mind which is in equilibrium, as Bacon says , 



m ' Discourse on the Study of Natural Philosophy,' p. 151. 



II Ibid. p. 229. o <]S"ovum Organum,' bk. II. Aphorism 36. 



